harw00d's post in Hd500 4cm With Midi Switching? was marked as the answer
I've already read the handbook and experimented myself.
You can send midi when in ABCD mode as each patch, when switched to will send a pgm change.so 1A will send PGM001, 1B will send PGM002 etc. This can not be altered.
If your amp has midi and a midi learn function, you can teach it what the midi PGM is for each patch and in turn control amp channels.
I had a Bugera TRIREC but its gone now.