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

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.

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 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. 

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.

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.