So far the hum issues are related to ground loops, which are a risk no matter what hardware you use. Many receivers and source components aren't as susceptible to it because they don't use three-prong power cords, while big power amps typically do and are therefore often the trigger for ground loop chases. Unfortunately for them, they are usually not the culprit - cable TV is the primary source of ground loops, due mostly to problems with the cable installation and the way it ties to the house's electrical ground. There are inexpensive methods of fixing such ground loop problems, so I would not let the risk of a ground loop prevent you from moving to a good amp like the 7700.

Balanced interconnects are not necessary to get a good, quiet system - plenty of us have had great success getting dead-quiet setups with standard unbalanced cables. I wouldn't get rid of the Yamaha just for the sake of having the option to use balanced interconnects. On the other hand, the 990 is a great piece of gear that offers some nice features the 3300 lacks (DVI switching, Pro Logic IIx, more robust bass management, video transcoding from s-video and composite to component, a third component input).