The recent addition of a Direct TV HD STB and BluRay disk player has me chasing hums again....*sigh*
I think I have this one sorted out, but I'll have to settle for "good enough" with the damn STB. The Panasonic BluRay was introducing some minor hum--I cured that by going Toslink into the Benchmark DAC--all good, quiet all the way to max volume (CDP and DVD share the "CD" input on the PRE; the HDTV goes to the "AUX" input on the PRE).
Now, the STB. Believe it or not, it's not your typical satellite/cable feed hum. Unplugging the coax feed does nothing, the hum is still there. I think the STB just has what they call "dirty power." I actually got better results (less hum) by running the TV audio out from the HDTV via RCA's to the preamp.
So now I've got the CDP and DVD perfectly quiet, the satellite signal fairly quiet (hearing the hum requires almost maxing the pre's volume). Just a little disappointed that they can't build a quiet damn box. I think the problem is, both the STB and the BluRay player use the 2 prong AC plugs, no dedicated ground. Everything else in the system is 3 prong, and quiet. Then again, if it was ground loop hum, it would pass it on to all of the connected equipment at the preamp--which it isn't. Must just be a noisy STB?
You'll note in the diagram that everything except the power amp is plugged into a common grounded Panamax 4300--so I don't have mixed potentials there, and the amp/pre when running by themselves are dead-quiet--so using the amp plugged direct isn't causing any mismatch.