Tough call; I just installed my PN64D550, and can't imagine the picture getting any better; it's stunning. that said, I didn't want the features of the upper-scale models such as the newer 8000's, and the hum you mentioned IS there, and is grating on my nerves. It eminates from the front of the panel, and is clearly audible at low-normal listening levels. I don't know if that is also the case for higher end, new Samsungs.
If you love the 8000 and can make use of its features, I would take the $ and put it towards a higher end model.