Hmm...I've had too many ciders to really work out what has already been said wrt to cross-overs etc, so I'll just add my own input, secure in the knowledge that Tony probably beat me to it

The empeg port is the same as a PC.
The sled port is the opposite.

But don't forget the caveat about the extra signals on the sled port. In particular, pin 4 has 12V on it, which may or may not have some effect on serial comms... so in this respect, making your own cable might be prudent.

Tony, I'm going to throw a wild guess here, and suggest that the overlay patch might need some work done on it to make it truly volboost compatible. Do you have any volboost parameters set?
(This would typically only be seen in tuner/aux modes - although IIRC volboost allows mp3 playback to be boost or cut, I could never see any motivation for doing so - the point of volboost was to allow tuner/aux volumes to be boost/cut to match the mp3 volume, not the other way around, so I doubt anyone would actually set volboost_mp3)

I don't know about how you are testing, but you could try setting ;@AC fake_tuner=1 (check syntax!), and seeing if that helps nay in debugging.