Yes, I use a single Rel T/7x with mono blocks. My hybrid mono blocks are balanced (+/- referenced to each other and not to ground), so I connect as shown for “Differential (i.e. Fully Balanced)” in the accompanying Rel booklet: Yellow connects to left + and Red to right +. I connect the Black wire to a screw on the power strip which is earth/ground.
This wiring also works for a pair of balanced PP 2A3 amps I built with no global feedback and floating secondaries. However, to eliminate hum with these amps I also had to connect both secondary commons to earth/ground via the same screw on the power strip.
I can’t say if this wiring works for all mono block amplifiers, but it works for me. You may have to experiment to get to get the lowest noise and functioning connection. The Rel documentation says that not connecting the Black wire may work better for some implementations.