Let me add my 2 cents on the P300...I've had mine a year and couldn't part with it. It's worth mentioning that the P300 has a RF coax in/out connection on the back (which isn't even mentioned in the owner's manual) that apparently is some kind of isolation device. I route my cable TV through the P300 using those connections, and it completely eliminated my hum problem. If you decide to buy used, make sure you're getting the multiwave option...it makes a significant improvement.
Hum and the PS Audio P300
I've had a hum in my system for several years. Its no mystery. Its caused by the different ground between my stereo (house ground) and the grounding of my Cable TV's (AT&T Broadband)ground. I've tried spliters and the little box with cable connectors on each end and a dedicated ground. None of that works. I upgraded from a ARC LS2B to a Sonic Frontiers Line 3 and the hum dropped 10db so I've lived with it. The question is I'm considering the purchase of a PS Audio P300 power plant for my CD and preamp's power. Can I expect the hum to go away or get worse? I can terminate the hum totally if I either disconnect the TV cable or isolate the DVD player which is connected to the TV from the preamp.