Connecting monoblock tube amps to single subwoofer.

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.  :).   

Thanks to all for the suggestions. I will be tinkering with various connection  options and will provide update afterward.

I figured REL knows their products a lot better than I do. 

Possibly.

Connecting a sub to the speakers is generally a bad idea. It's an archaic Band-Aid.

Connecting to one speaker only ½ works for mono bass such as on LPs. Master tapes do not have mono bottom. 

With a separate power amplifier or pre-out/pwr-in on an integrated, much better results can be obtained by rolling the bottom octave[s] out of the mains with a simple passive first order filter and driving the sub from line level.

Bottom octave[s] are slightly above the anechoic -3dB frequency.

Connecting to one speaker only ½ works for mono bass such as on LPs. Master tapes do not have mono bottom. 

@ieales 

At 80Hz, the waveform is 14 feet long. The ear cannot know a tone is there until the entire waveform has passed by it, and to acknowledge the frequency takes a couple more iterations.

By this time, unless your room is enormous, all the bass in the room below 80Hz (and probably higher too) is entirely reverberant!

Consequently the fact that the bass is mono below 80Hz is of no consequence whatsoever, unless you play headphones in which case no need for a subwoofer 😁

As long as the bass is mono on an LP, (for the few milliseconds where this might be the case, should there be out-of-phase bass in the groove) its of no consequence there either.

However I prefer using the preamp output if possible. But for the subs its still run mono.