Connecting monoblock tube amps to single subwoofer.

Hi all,

I have added a REL5TX sub to my system which is driven by VTL225 monoblocks.Currently the sub and amps are connected via the low level outputs on my preamp. I would prefer to use the high level connection to the REL by direct  connection to the the VTLs. I am concerned about creation of a ground loop that will cause hum on the system. I have been told that the ground loop can be avoided by running a ground wire directly between each amp. Is this a possible solution, and if so, where / how should the ground "jumper" be connected to each monoblock?

Many thanks in advance for any comments or suggestions.


YOu should find your answer within the following. REL does a really good job of providing a lot of how-to information.


I went through exactly this question. REL basically warns people away from doing this, but I got expert advice on Audiogon from Almarg (RIP), that the high level connection can work.

I have two quicksilver tube monoblocks. I connect the REL speakon wires thus:

RED wire to RIGHT POSITIVE on Monoblock 1

YELLOW wire to LEFT POSITIVE on Monoblock 2

BLACK wire to LEFT NEGATIVE on Monblock 2.

There is no hum, for me. YMMV. If I were you, I'd contact VTL. 

Please see this discussion:


Please let us know what you think after you've tried it. Though REL has pushed hard on the idea of speaker-level connections, I am doubtful that it provides any demonstrable benefits.

@mike_in_nc: In fact, the exact opposite may be true. Why add the commonly-admitted limitations of tube amp output transformers to the signal if not necessary?

Brian Ding of Rythmik Audio offers his subs with plate amps that include provisions for both high-level (power amp) and line-level connection. He prefers and recommends line-level for best sound quality.

If hilde’s suggestion doesn’t pan out you could always follow REL’s advice and get another T5/x.  They repeatedly advocate for 1 sub per amp.  Also, what is your preamp?  Does it have a summed mono line output? You shouldn’t just Y-connect stereo outputs. 

I run a carbon special on my Pass mono's.  I just left the ground wire floating and wired left/right to each amp.  I have zero hum/noise.

High level connections are a bad idea.

Any time there is a loop, there is a potential for problems. The main amplifiers could be oscillating above the audible range at a low level.

Ideally, you should be rolling the low end out of the mains. This gives an effective power increase to the mains and simplifies getting the phase correct between mains and subs.

When I was running a pair of mono tube amps, the tech I contacted at REL was adamant that I run my single T5 in mono, from a single amp only. The red and yellow wire twisted together and attached to the 8-ohm speaker tap and the black wire to the ground. I saw no reason to go against his instructions.

I figured REL knows their products a lot better than I do.  :).   

Thanks to all for the suggestions. I will be tinkering with various connection  options and will provide update afterward.

I figured REL knows their products a lot better than I do. 


Connecting a sub to the speakers is generally a bad idea. It's an archaic Band-Aid.

Connecting to one speaker only ½ works for mono bass such as on LPs. Master tapes do not have mono bottom. 

With a separate power amplifier or pre-out/pwr-in on an integrated, much better results can be obtained by rolling the bottom octave[s] out of the mains with a simple passive first order filter and driving the sub from line level.

Bottom octave[s] are slightly above the anechoic -3dB frequency.

Connecting to one speaker only ½ works for mono bass such as on LPs. Master tapes do not have mono bottom. 


At 80Hz, the waveform is 14 feet long. The ear cannot know a tone is there until the entire waveform has passed by it, and to acknowledge the frequency takes a couple more iterations.

By this time, unless your room is enormous, all the bass in the room below 80Hz (and probably higher too) is entirely reverberant!

Consequently the fact that the bass is mono below 80Hz is of no consequence whatsoever, unless you play headphones in which case no need for a subwoofer 😁

As long as the bass is mono on an LP, (for the few milliseconds where this might be the case, should there be out-of-phase bass in the groove) its of no consequence there either.

However I prefer using the preamp output if possible. But for the subs its still run mono.

It's not about wavelength. It's about origin. I had single subs for HT and HiFi for about 15 years before I got duals. In both cases the subs were well integrated and time corrected with REW.

There is no comparison.                        

Dual subs are closer to control room large monitors than any single sub can ever hope to be.

The answer to the question is to add a second REL T5x to the system. This way, each sub can run off the particular mono block. This is REL's advice and it is well given. The addition of a second sub to my system is a nice benefit when it comes to overall bottom end resolution. 

One thing to consider, the addition of a second sub is exponentially harder to set up, with the blend and the correct positioning than just one sub. Given enough tweaking it works out well, but time and patience will be required to make it work.

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ieales's avatar


953 posts


I figured REL knows their products a lot better than I do. 

@ ieales:




Oh no sir, Not "possibly" but absolutely.  I can assure you that REL does indeed know their product a lot better than I do.  :)

One more thought which has usually served me well is to apply the principle of  Occam's Razor.  I paraphrase: "Generally the simplest answer is the best answer."

Not always. of course, but often enough. YMMV.  :)  And  of course, some anal-obsessive audiophiles are likely to surely disagree.  :)

OTOH, I'm with daveyf, I eventually got a second sub, even before swapping my mono tube amps for a tube integrated.

the addition of a second sub is exponentially harder to set up, with the blend and the correct positioning, than just one sub.



There are but two kinds of people in this and other worlds, the yup's and the nope's

Connecting monoblock tube amps to single subwoofer

Hello to all, and Happy Friday.

I connected the high level outputs of the VTL monoblocks to the REL5TX. The high level connection was made by running the red and yellow wires separately to the positive terminals to each monoblock, and connecting the black wire to the negative terminal on one of the amps.Happy to report that, at least in my system, no hum. Results may well be different with other components (I run the VTL MB 225 monoblocks, driven by a Rogue 99 Magnum pre) Completely happy with the performance, and hope this may be a solution for others with a similar setup.

@ps   +1000



@ieales   I guess that depends on how you want a pairing to sound, if you just plop them down anywhere, yeah you are right... no problem. ( Nope). OTOH, IF you want an invisible blend with your mains...then yup! 🤔

I've been setting up subs in systems for decades. Today with room correction and tools like REW, setting up two subs is actually easier than one.

My room is ±5dB from 20Hz. Show me a plot of your room and then we'll talk.

Assuming the op means he has a T/5x, setup is going to be very, very difficult:

  • No continuous phase control
  • No polarity inversion
  • No crossover order
  • No delay

If he has ported mains, fuhgeddaboudit.

See Subterranean Homesick Blues Guide to Subwoofers

As a composer once said of one of my systems:

"Those little speakers aren't putting out all that gorgeous bass?!?!?!?!?"

- No, there's a Force subwoofer at the end of the sofa. - 

"Every other subwoofer I've ever heard just boomed!"

What we are talking about is two REL T5x’s. They do have a phase adjustment switch and adjustable xover settings. Nonetheless, I think you are agreeing with me that the set up is not easy. I am no fan of digital room correction tools, much prefer to just use my ears.

I may be dead wrong here but I was always told that the bass signal output is mono. This would mean that when running mono blocks you can tap the sub off just one amp and be ok. The key to this is where you cross over the sub at.

@sgreg1 Bass may be omnidirectional, which is a property of our hearing, but there is NO guarantee from recording to recording, that the signal below 80Hz has been combined and evenly distributed to the L&R channels.

The mastering engineer must chose to do this, and we have no idea when or if this ever happens.

In other words, bass may not be heard as directional but the bass in a recording is not necessarily monophonic.

What we are talking about is two REL T5x’s. They do have a phase adjustment switch and adjustable xover settings.

OOPs. The 0-180 switch is hidden in the logo. A 0-180 switch is the equivalent of moving the sub 14 feet @ 40Hz.

The XO control is frequency only. No filter type or order.