Connecting monoblock tube amps to single subwoofer.

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.