I have noticed that when I (manually) advance the volume control quickly, I sometimes hear clicks. Are the commands that you are sending (i.e. changes in volume) small steps (i.e. 1-2 dB) or large steps? I only ask because if I smoothly advance the volume control, I do not hear clicks.

On the other hand...the RS-232 interface does rely upon signals that are akin to impulses (digital strings at a high baud rate)...so that *could* be part of the reason...and I would start by checking the shielding of that cable. Then again, that it does it with IR commands suggests otherwise.

As far as one of the balanced outs no longer working, you could employ a high-Z to Low-Z impedance transformer going from the RCA output (to the "High-Z" side of the transformer), and the XLR side going to the balanced input on the 7700. At least this would convert the RCA single-ended signal to a balanced one.