REL subwoofer using only red cable not yellow per REL technical dept

adding a link. https://relsupport.zendesk.com/hc/en-us/articles/115004649048-Stereo-Subs-Connection-Method

Here it says you can use either red or yellow.

Going back to OP.  I have two REL T/9i in my setup.  I had them wired with yellow left and red right for both.  Then, I read about RELs recommendation to run them as stereo, and wired them that way.  

They suggest that you can put red and yellow together, such that you are running two channels in each sub, as they are designed.   They also suggest that if the gain is too high, and you have to turn the sub's volume low, then you can lift the yellow.  In my case, I had to run the RELs on only one or two clicks above zero, or they'd be too loud.  

Now, I have the red wire on each sub connected to one channel (i.e. one on left and one on right), and the yellow wire is not connected.   In this configuration, the volume is between 10 and 12 o'clock, and I ended up setting crossover higher than I expected.  This ended up bringing out the midrange much more clearly, and the overall sound stage opened up.  My experience matched yours.  I did also cover the bare wire to prevent any sort of short or other issue.  

Thank you Jacob.

It isn't a 100% certainty that hum will occur when connecting a single subwoofer high level to two monoblocks, though in our experience it is pretty common, and when it does occur it can be near-impossible to eliminate. 

@jacobatrel 'Jacob at Rel', thanks for joining the discussion.

Is it 100% certain, regardless of what monoblocks are being used that the low level hum will exist, or is this a strong recommendation just in case.

I'm really not concerned about having mono vs stereo imagining. I've had mono and dual subs in my system in the past. I agree two is better than one. I don't use a preamp, so the low level input isn't possible.

Ground hum will occur when connecting a single subwoofer to two monoblocks, and this hum can be extremely difficult if not impossible to eliminate. If you are only able to support a single subwoofer in your system at this time you can connect it to only one of the monoblocks, though only that channel which the subwoofer is connected to will have low-frequency extension, which can throw off the stereo imaging of your system. When using a single REL subwoofer with monoblocks it is recommended to use the Low Level input on the sub, connected to preamp outputs, which will allow the single subwoofer to receive both left and right channel information.

I had a low level hum that was driving me nuts with checking tubes and various possible sources when finally I realized my 2 RELs weren't on the same page. One had the ground lifted with a little switch, the other has no switch...I lifted the ground on the second REL and instantly all was quiet.

"....REL suggested first unplug yellow wire from speaker terminal and turn up sub gain as that might improve the bass integration...."

Do NOT leave the yellow wire floating! I recently traced a low-level rumbling sound to an open connection between the yellow wire and the Left Speaker input inside the Speakon connector. 

And that's why I only want to run a single sub, it should be plenty. I really don't even need a sub for 2 channel, but I know it will enhance the experience somewhat. I do however require a sub for home theatre. I just wish someone with Pass Labs monoblocks would disconnect one sub from their system to see if there's any low level hum. Is Rel Acoustics saying you need 2 subs as a CYA, or is it 100% guaranteed the hum will exist when running monoblocks with a single sub?

REL does indeed recommend two(2) subs if you are using mono block amps. I have mono blocks, so two subs it is. However, the issue becomes that depending on room size, two subs can easily overload your room. In my case, my room is very small, albeit with a vault ceiling ( which adds to volume), so the little T5i is the maximum that I can get away with in this room. I think one has to consider the room/sub interface as a major factor when using two REL's. If you utilize mono blocks and your room is like mine (11x10) or smaller...and without the vault ceiling, I would suspect that you will run into trouble with the various room nodes you are going to excite.

What is a “faster” subwoofer?

@wolf_garcia - yes, I got that from their website:

Here they’re talking about "Balanced/Differential/Class D" connection.

When doing so, please combine red and yellow leads together to form a single composite “+” or “hot” conductor as shown in the diagram below. Do not connect the black wire to the output terminals. Instead connect it to a chassis bolt as shown below.

However, they go on to say:

We do not recommend use of a single REL with ANY brand of monoblock amplifier as it is possible to observe slight channel-to-channel dc offset that can result in mild oscillation. For any monoblock, we recommend use of stereo pairs of the appropriate model of REL sub.

I’ve read issues with some Macintosh monoblocks, and I’m asking specifically about Pass Labs monoblocks with regards to connecting a single Rel sub to one of the monoblocks (using high level connection into sub).

I sum the 2 positive leads from each of my 2 RELs together in a pair of AQ banana plugs, and the negative leads into 2 other bananas and stick ’em in the power amp. The RELs are used in mono with this configuration. The main speakers are spades in the same 5 way binding posts and they all fit and work perfectly...I built the REL cables from Canare 4S11 "star quad" (using 3 of the 4 wires in each cable) with angled Neutrik plugs from Liechtenstein.

Yep, they have the "Subwoofer Finder" tool, and suggest what sub(s) to use, but don't mention anything in that tool about using monoblocks, they only ask what type of speakers. Lots of helpful information on that site, and it would be worthwhile reading some of it prior to making a purchase.
I was hoping someone like @karmapolice would be willing to disconnect one of his Rel subs from one of the monoblocks and report back if there is any hum (assuming he still has rel sub and pass labs monos), or anyone else for that matter. 

Wow, this is great info.  I just returned a REL based on my experience with one click making too much of a difference.  For instance, 7 clicks wasn’t really audible, 8 clicks was too much.  I figured I had bought too much sub for my little totems.  I recently realized that there is a speaker / recommended sub section on RELS sight.

Would anyone know if there’s a hum using a single Pass Labs monoblock
with a single Rel Subwoofer via speakon connection? Rel Acoustics claims you need to connect 2 subs when using monoblocks.

Please excuse the post from millercarbon, he is incapable of recognizing any other subwoofers beyond his ‘swarm’ sub system. He can’t seems to understand the simple fact that most people don’t have room for 4 subs, no matter how good they sound.

REL also provides assistance with some very informative information on YouTube which might also apply to many other brands of subwoofers. https://rel.net/how-to-connect-my-rel/

not sure what you mean but since I spoke to REL technical department about it seems they would know best.........

Do you REL guys have any idea what you're doing? Or is it all colors right on down the line?

Yes daveyf you are correct that is the correct default setup but REL is telling me that if I connect only the red cable I can raise the gain and the sound might overall sound better

I am under the impression that if you use two REL subs and hook them up to mono blocks...one sub per channel, you should sum the red and yellow wires together and attach the pair to the positive terminal and the black wire would go to the negative terminal...for each sub per mono block. This is how I set up my two REL's with my tube mono blocks.
Not sure if this applies to a balanced design mono block.