Connecting monoblock tube amps to single subwoofer.

I went through exactly this question. REL basically warns people away from doing this, but I got expert advice on Audiogon from Almarg (RIP), that the high level connection can work.

I have two quicksilver tube monoblocks. I connect the REL speakon wires thus:

RED wire to RIGHT POSITIVE on Monoblock 1

YELLOW wire to LEFT POSITIVE on Monoblock 2

BLACK wire to LEFT NEGATIVE on Monblock 2.

There is no hum, for me. YMMV. If I were you, I'd contact VTL. 

Please see this discussion:

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.