Connecting monoblock tube amps to single subwoofer.

@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.  :).