In the message above, answering Paladin, Ritz wrote:

The common thread seems to be hum and ground loop issues. You might want to check out the 7500/7700 forum before making that buying decision. It could be nothing...or it could be a sign of a real problem.

I have been "playing with" or "debugging" this issue. It is absolutely NOT the fault of the amp.
Amp is dead quiet when not connected to Pre/Pro. Slight hum this AM when connected unbalanced. So I tried some old MIC cables (XLR(F)-XLR(M). Voila...no hum!!! Now I'm not stating that this is the answer. There is another variable and that is that the amp had been on for an hour this AM.
So I'll eliminate that factor by leaving 7700 off for a few hours. Then if the balanced connections continue to be associated with dead quiet, I think I can draw the obvious conclusion.

So in response to Ritz: the amp ain't the problem.
In response to Paladin: I use electrostats also, up front L & R. I also use a center that is not timbre matched. I am delighted with the 990-7700 combo...AND YOU CAN'T BEAT THE PRICE !!!