So I've tried this out multiple ways.

Channel 2, flat, I get no sound out of the sub at all. 568 is set up with center as small 24.96, sides as small 24.96 and sub as mono. This is set in the legacy application as the arcane remote setup is... batty.

Sub's SPDIF is coming out of the center. Center is also setup as controller with M5 lead to it, then comms output to 511, all other speakers branch from 511 including sub. BTW the sub's comms ports aren't labeled in/out nor does it have a serial port.

And yes the 33's have those weird left/right switches that are indeed backwards. That's how I managed to have the 33's setup all weird before I got the sub... now stuff comes out of the correct 33's.

I'll have more time tonight to experiment but I get the sense this is something really obvious that I'm missing.

main room: 861v4, 2x6000L/R (24/96) 1x5500HC (24/96), 2x DSP33(Rears)
MS200/Roon, 598, Revox b710mkII tape deck, Revox b760 FM tuner, Revox b790 turntable
bedroom: 568.2mm, 3x5000(LCR) (24/96)