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....