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

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. 

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.

@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).

What is a “faster” subwoofer?

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.