The difference between 7700 and 7200 is essentially just the balanced design, which affects the topography of the seven amp channels. The difference between the 7700 and the 7500 is that the 7700 has seven balanced channels and the 7500 has five balanced channels.

Oh, and while I currently still have a set of Outlaw PCB cables between my 990 and my 7500, I initially used a set of five microphone cables ($5 each) and they worked just as well. I would have left them, but the ones I had were too long (got in the way of everything else) and the connectors were not as smooth as the PCB connectors (one of them really wanted to stick in place).