Thought I should give an update as fellow Outlaws Gonk, AvFan and KOYANN were nice to offer suggestions. First, turns out I was wrong about one portion of the problem, that being the 5.1 analog inputs being connected then disconnected from the 990 did in fact cause hum to be present when connected to the DV-45. Gonk, I did try your idea and connected an optical audio cable between 990 and the 45 with no other audio connections. There was no hum. While this was good info, I didn't want to lose the 5.1 feature.

I then decided to explore AvFan's ground loop thought further. While I was sure some of these power combinations I'd tried before, since I had the rack apart, I might as well do more trials. Turns out, if all electronics went through the Panamax 5100 the hum was all but gone. Panamax was plugged to the dedicated 20 Amp circuit.

Now enters the Kuro PRO111FD! (Wow what a panel!) Anyway, had everything back in place, and the Kuro going into the Panamax - now another (type) of hum appears! This time not from the speakers but from the 7500! This was different than anything previous, and it cycled up for 5 seconds (loud) then down for 15 seconds then back up for 5 seconds. A very low frequency hum. It was obvious the new Plasma and the 7500 weren't getting along!

GREAT, now what. I pulled the rack out again, (each time having to remove the 7500 as the rack won't roll with it in place), and again tried various circuit combinations with extension cords. I remembered a thread from a while ago talking about the relationship of the house circuits to the bus bars on the electrical panel. After studying my electric panel's circuit layout (200 Amp , 40 breaker panel), I realized all the circuit combinations I was trying were coming off the same bus. I then took the 7500 to a different circuit/bus and kept the Kuro plugged into the Panamax. The new hum went away! As of today, this setup seems to be "holding". Only down side is the 7500 has no protection.

I felt I should share this ordeal with fellow Outlaws as it sure has been a frustrating effort, and hopefully others may be able to avoid similar issues. Look forward to any follow-up thoughts from the gang.