Connecting monoblock tube amps to single subwoofer.

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.

It's not about wavelength. It's about origin. I had single subs for HT and HiFi for about 15 years before I got duals. In both cases the subs were well integrated and time corrected with REW.

There is no comparison.                        

Dual subs are closer to control room large monitors than any single sub can ever hope to be.

The answer to the question is to add a second REL T5x to the system. This way, each sub can run off the particular mono block. This is REL's advice and it is well given. The addition of a second sub to my system is a nice benefit when it comes to overall bottom end resolution. 

One thing to consider, the addition of a second sub is exponentially harder to set up, with the blend and the correct positioning than just one sub. Given enough tweaking it works out well, but time and patience will be required to make it work.