Couple of random thoughts;
- Did you have this issue previously - you mentioned hiss but not hum?
- Is it possible to check the ground at the receptacle?
- You mentioned isolating to the Denon, but have you removed all inputs (dvd player and all cables) from the Denon and checked? I would also remove the trigger and check for the hum. If you find the hum is gone, begin adding the cables back and see when/if it returns.
- is the dvd player connected to the same receptacle as the denon and amp?
- Do you have another set of RCA cables to test with? Highly unlikely, but I have experienced stranger things....
The noise sounded different before, but I think there are a few explanations:
-I first heard it when testing the ADA with M3's (lacking a midwoofer, and less sensative vs the M5's and VP180 that I have hooked up now)
-I tested the ADA/M3s in my office, which sounds different than my basement (9x11x9 office, vs 20x13x7.5 basement)
-I only had LCR channels enabled on my Denon at the time, so I'm guessing the internal power supply didn't have to work as hard as it's working now (with full 7.2.4 enabled)
I don't have another set of cables handy, but am working to acquire some, along with a basic receptacle tester