Last May I purchased a 7500 to replace some older amps that I had built myself. Don't get me wrong these amps I built were awesome, just that they were too big to fit in the newly acquired equipment cabinet. In the process of doing the upgrade, I decided to replace the old audio cables. I acquired these cables from a major midwest distributer and spent a fair sum of money on them. When the 7500 arrived I hooked it up to the 990 and I had a constant low level hum from the speakers. If I muted the 990, the hum went away. I called Scott after trying to fix this for some time with no luck. He said that he would be willing to help me out, but I wasn't ready to do that yet and decided to try a number of things. Since I work in Seattle, this was limited to the occasional weekend that I was home and had no other chores to do. Long story short, I replaced the audio cables with Blue Jean's cables, and the hum went away. Out of curiosity, I did an autopsy on these other cables I purchased and found that they had a red wire connected to the center connector and a white wire to the common. The shielding was not connected at all. Go figure!