HA!!
after three years, I have finally realised I had made a gross mistake in my commands to set the "record monitor" switch!
I have been piqued by adriaanhendrik success with his amazing controller, and decided to get at the bottom of this impasse.
I searched for my notes (which I did not find...), took the CBX off its shelf, and made it to work after some frobnication with my PCR-300 controller.
(one annoying quirk I discovered in the PCR editor: when you assign sysex to a controller, the bytes to send are written in Hex, but the variable's limits are in decimal!)
So now I guess I can resume work for that CBX controller...
P.O.