I just discovered this page and am very interested in how you are progressing in terms of getting it to work.
I am dealing with something similar, albeit in a different environment, namely a Crestron home automation system. There is a module available from Crestron to control the SR5500 (in my case) serially and it is working fine, apart from the MultiSpeaker outputs, which I have to control via Infrared.
The MSP commands in the Crestron module (programmed using a proprietary language called SIMPL+) look identical to yours, so I was wondering, have you managed to turn the MultiSpeaker outputs on and off, select source, and so on?
My setup is correct (ie Rear Surround speakers set to NONE), but the response to all MSP commands is "15" indicating NAK.
If you had any thoughts or recommendations to share, it would be greatly appreciated!
Kind regards Marc