REL No.31 Reference Series Subwoofer

The REL No.31 is a high-performance active subwoofer developed for reference stereo systems, premium home cinemas and installations requiring deep extension with exceptional speed and control.

It follows the same core design philosophy as REL’s flagship No.32 but uses a more compact cabinet, 300mm driver and 900W amplifier.

The No.31 can operate as a single subwoofer, as part of a stereo pair or within a vertically stacked REL Reference line array.

Reference Performance in a More Manageable Cabinet

The No.31 was created for customers who want the construction, adjustment and performance of REL’s Reference range but cannot accommodate the considerably larger No.32.

Although described as compact within the Reference range, the No.31 remains a large and exceptionally capable subwoofer.

Its lower profile and reduced cabinet volume make it easier to integrate into a wider variety of listening rooms while preserving the scale, finish quality and advanced controls expected from a flagship design.

300mm Pure Carbon-Fibre Driver

The No.31 uses a front-firing 300mm long-throw driver constructed from pure carbon fibre.

An inverted carbon-fibre centre cap reinforces the main cone, creating a stiff, lightweight structure capable of reacting quickly while remaining controlled during large low-frequency movements.

The driver is mounted within a rigid die-cast alloy chassis that provides a stable mechanical foundation for the moving assembly.

The combination is designed to produce the speed and texture required for music while retaining the excursion and power handling needed for demanding cinema effects.

75mm Voice Coil and Substantial Motor System

A specially wound 75mm voice coil is driven by a magnetic structure weighing approximately 13 pounds.

The motor system is designed to keep the voice coil securely controlled within the magnetic gap, including during demanding passages and high-output operation.

This allows the No.31 to combine considerable physical impact with the delicacy required to reproduce subtle changes in pitch, texture and timing.

Closed-Box, Front-Firing Design

The No.31 uses a sealed cabinet with no reflex port or passive radiator.

Air trapped within the cabinet helps control the driver’s movement, supporting a fast response and gradual low-frequency roll-off.

The absence of a reflex port also removes the possibility of port airflow noise during demanding low-frequency passages.

Its front-firing configuration allows the cabinet to operate on a variety of firm floor surfaces without requiring operating clearance beneath a down-firing driver.

17Hz Low-Frequency Extension

REL specifies low-frequency extension to 17Hz at minus 6dB.

This allows the No.31 to reproduce extremely deep musical and cinematic information while adding greater scale and authority to the entire system.

Room dimensions, positioning, seating location and the settings used on the subwoofer will all influence the extension achieved at the listening position.

900W NextGen 7 Class D Amplifier

The No.31 is powered by a 900W RMS NextGen 7 Class D amplifier.

The core amplifier design is closely related to the 1,000W unit used by the larger No.32, with its output matched to the No.31’s 300mm driver and cabinet.

This provides the current delivery and dynamic headroom required for sudden musical transients and powerful home cinema effects.

REL rates the amplifier at continuous RMS output rather than using a short-duration peak figure.

LimitLess Limiter

REL’s LimitLess Limiter is designed to allow the amplifier and driver to operate close to their intended performance limits without introducing obvious compression.

Rather than making loud passages sound restricted or mechanically strained, the system helps preserve contrast, timing and a sense of ease.

Protection remains active behind the scenes, helping the No.31 deliver substantial output while preserving the long-term reliability of the amplifier and driver.

Fully Electronic Protection

REL’s electronic Set-Safe protection monitors the amplifier during operation.

Additional protection is provided against DC faults and output short circuits.

These systems are designed to operate automatically and do not replace sensible installation, ventilation and connection practices.

Dual Parametric Filters

Two adjustable parametric filters allow the No.31 to be fine-tuned more precisely for the room and the connected loudspeakers.

One filter can be used to refine the deepest part of the subwoofer’s response, while the other can assist with the transition between the subwoofer and main speakers.

They can also help reduce the effect of a specific room-related bass excess without applying broad correction across the entire low-frequency range.

Parametric filtering should be treated as a final adjustment after sensible positioning, phase, level and crossover settings have been established.

REL Reference Remote Control

The supplied REL Reference Remote allows the principal setup controls to be adjusted from the listening position.

Separate controls are provided for High Level or low-level gain, crossover frequency and the dedicated .1/LFE channel.

Metal toggle switches provide access to phase and the two parametric filters, allowing changes to be assessed without repeatedly moving between the listening seat and the rear of the subwoofer.

Once the No.31 has been correctly adjusted, the remote can be set aside until the room, equipment or system layout changes.

REL High Level Connection

REL’s preferred connection for many conventional stereo systems is the supplied High Level Neutrik Speakon cable.

This connection receives the same signal and sonic character being delivered by the main amplifier to the loudspeakers.

The main speakers remain connected directly to the amplifier and continue to receive their normal full-range signal. The No.31 is added alongside them and adjusted to support their natural low-frequency roll-off.

A 10-metre nominal Neutrik Speakon High Level cable is supplied with the No.31.

Stereo Low-Level RCA Inputs

Two RCA inputs allow the No.31 to receive a stereo low-level signal from a suitable pre-amplifier, integrated amplifier, processor or active loudspeaker system.

The source must provide variable outputs that follow the main system volume.

Low-level connection can be particularly useful with active loudspeakers and equipment that does not permit a suitable High Level connection.

Dedicated RCA LFE Input

A dedicated RCA LFE input connects to the subwoofer output of a compatible AV receiver or surround processor.

This carries the separate low-frequency effects channel used by film, television, concert and multichannel game soundtracks.

The No.31 provides independent .1/LFE level adjustment, allowing its cinema output to be configured separately from the High Level or stereo low-level connection.

Balanced XLR LFE Input

A balanced XLR LFE input is provided for compatible surround processors and professionally planned installations.

Balanced connection can be useful where the processor is positioned a considerable distance from the No.31 or the cable route passes close to other electrical equipment.

The connected processor must provide a suitable variable balanced subwoofer output.

Simultaneous Music and Cinema Connection

The No.31 can use its High Level or stereo low-level connection for music while also remaining connected to the dedicated LFE output of a home cinema processor.

The two signal paths provide separate gain adjustment, allowing the subwoofer to integrate naturally with the main speakers during stereo listening while reproducing the dedicated .1 channel during films.

This arrangement is particularly useful in systems where the same main loudspeakers and power amplifier are shared between stereo and home cinema playback.

Signal Output Connections

The No.31 provides High Level Speakon, low-level RCA, LFE XLR and line-level XLR outputs.

These connections can be used to pass the relevant signal to an additional compatible REL when building a stereo pair, multiple-subwoofer system or Reference line array.

Every No.31 remains independently powered and retains its own level, crossover, phase and filter adjustments.

Airship II Wireless Compatibility

The No.31 is compatible with the optional REL Airship II wireless system .

Airship II can transmit High Level and LFE signals using REL’s dedicated wireless connection, providing greater freedom when a conventional cable route would be difficult.

High Level and LFE signals can be carried concurrently, preserving the No.31’s ability to operate differently for stereo music and home cinema.

The Airship II receiver is designed to mount directly to the amplifier panel of compatible Reference and Serie S subwoofers, including the No.31.

Bassline Blue High Level Cable

A standard 10-metre High Level cable is supplied with the No.31.

The optional REL Bassline Blue High Level cable provides an upgraded connection for compatible amplifiers and REL subwoofers.

Bassline Blue is available in different versions and lengths, including configurations developed for compatible Naim amplifiers.

Confirm the correct cable version and amplifier wiring arrangement before ordering.

Positioning the REL No.31

Begin with the No.31 positioned along the same wall as the main loudspeakers.

A corner can increase output and low-frequency extension, while moving the cabinet away from the corner may reduce excessive bass caused by the room.

The No.31 should not be enclosed tightly within furniture or positioned where its front driver, rear amplifier panel or ventilation can be obstructed.

Its 52kg weight and substantial depth mean that access, floor strength and the final cable route should be considered before moving it into position.

Initial Stereo Setup

1. Place the No.31 along the same wall as the main loudspeakers.
2. Switch off the complete system before making connections.
3. Connect the supplied High Level cable or suitable stereo low-level interconnect.
4. Begin with conservative crossover and gain settings.
5. Select the phase setting that produces the strongest and most coherent bass at the listening position.
6. Adjust the crossover so the No.31 supports the natural roll-off of the main loudspeakers.
7. Increase the gain until the added weight is clearly audible.
8. Reduce the gain slightly until the subwoofer no longer draws attention to itself.
9. Use the parametric filters only after placement, phase, crossover and gain have been established.
10. Complete the setup using familiar music from the normal listening position.

Initial Home Cinema Setup

1. Connect the AV receiver or processor’s subwoofer output to the appropriate RCA or balanced XLR LFE input.
2. Set the No.31’s dedicated .1/LFE gain conservatively.
3. Configure suitable loudspeaker crossover points within the AV processor.
4. Run the processor’s measurement and room-correction procedure where provided.
5. Review the measured subwoofer distance, level and crossover settings.
6. Check that the No.31 does not overpower dialogue or the main loudspeakers.
7. Complete the setup using familiar film, concert and music material.

Single No.31, Stereo Pair or Line Array?

REL Reference Line Arrays

The No.31 is designed to support vertically stacked Reference line-array systems.

A typical stereo Reference array uses three matching subwoofers beside each main loudspeaker, although exceptionally tall rooms may accommodate up to five units per side.

Every subwoofer within the stack retains independent crossover and gain adjustment, allowing the lower, middle and upper units to be configured for their position.

The purpose is not simply to produce more bass. A correctly designed array can increase the apparent height, scale and physical presence of the reproduced soundstage.

Reference line arrays require suitable room dimensions, dedicated hardware, careful signal distribution and professional planning.

Bassline Blue Line Array Connectors

The REL Bassline Blue Line Array Connectors can pass the High Level signal between compatible REL subwoofers within a vertical stack.

This allows one main High Level cable to run from the amplifier to the lower subwoofer, with shorter connectors carrying the signal to the units above.

A pair of connectors is required for each vertical level when building matching left and right arrays.

The number of connectors and mounting components required should be confirmed as part of the complete system design.

REL No.31 or No.32?

Both models provide Reference Remote control, High Level and LFE connectivity, dual parametric filtering, Airship II compatibility and support for vertically stacked Reference arrays.

The choice should be based on the room, main loudspeakers, desired scale and the physical space available rather than amplifier output alone.

Curved Reference Cabinet

The No.31 uses the curving cabinet form associated with REL’s Reference range.

The rounded sidewalls contribute to the cabinet’s visual character while avoiding the appearance of a conventional square subwoofer enclosure.

Plated handles are recessed into elliptical sections of the cabinet, providing secure lifting points when the substantial subwoofer must be moved.

The handles should be used with appropriate lifting assistance rather than attempting to move the No.31 alone.

String Grille

The front of the No.31 uses REL’s distinctive string grille rather than a conventional cloth-covered frame.

The open construction allows the large driver to move substantial volumes of air without creating the obstruction or noise that a tightly woven cloth grille could introduce.

It also gives the Reference model a recognisable visual identity while protecting the driver from casual contact.

Twelve-Coat Piano Black Lacquer

The cabinet is finished in twelve coats of piano-black lacquer.

This deep gloss finish reflects the time and craftsmanship involved in building the No.31 and helps it complement similarly finished reference loudspeakers and equipment.

Use a clean, soft microfibre cloth for routine care and avoid abrasive cleaners that could mark the lacquer.

System Matching

Browse our home cinema subwoofers , home cinema amplifiers , home cinema speakers , wireless accessories and subwoofer and system cables , or explore the REL range at Audio T .

Key Features

• REL Reference Series active subwoofer
• Closed-box cabinet with front-firing driver
• 300mm long-throw pure carbon-fibre cone
• Inverted carbon-fibre centre cap
• Die-cast alloy driver chassis
• Specially wound 75mm voice coil
• Approximately 13-pound magnetic structure
• 17Hz low-frequency extension at minus 6dB
• 900W RMS NextGen 7 Class D amplifier
• REL LimitLess Limiter
• Dual adjustable parametric filters
• Supplied REL Reference Remote
• High Level Neutrik Speakon input
• Two stereo low-level RCA inputs
• Dedicated RCA LFE input
• Balanced XLR LFE input
• High Level Speakon signal output
• Low-level RCA signal output
• Balanced XLR LFE and line outputs
• High Level and LFE operation for combined music and cinema systems
• Compatible with REL Airship II
• Suitable for single and stereo-pair systems
• Supports vertically stacked Reference line arrays
• Curved Reference cabinet
• Recessed plated lifting handles
• Low-restriction string grille
• Twelve-coat piano-black lacquer finish
• Fully electronic Set-Safe protection
• DC fault and output-short protection
• Three-year REL warranty, subject to current terms

Frequently Asked Questions