The following is a description of my experience with the dreaded hum and what I did to get rid of it.

I moved into a new apartment this weekend and finally got a chance to hook up all of my home theater equipment that I had purchased, but not yet opened.

For your reference, my equipment consists of:
B&W 804S fronts
B&W HTM3S Center
Monster 3250 Amp
Outlaw 990
Cobalt balanced interconnects
Coblat speaker wire
Denon 1920 DVD player
Monster HTS 3500

After reading many posts on hum, I decided to preemptively buy a Jensen VRD-1FF ( scroll down a bit ).

I hooked everything up the day before the cable company came out to install my service. Everything was fine - no hum at all. The next morning, I'm sitting in the living room watching the cable guy hook everything up. The second he touches the coax from the wall to the cable box, the wattage meters on my Monster Amp go from 0 to 5 or so - and the hum begins. The cable guy tells me that the cable is grounded correctly in the basement and that he doesn't know what could be causing the hum. I realized very quickly that he wasn't going to be much help.

So after he leaves, I hook up the Jensen isolator and the hum is gone. At this point, I'm patting myself on the back for having the foresight to buy such a fantastic little product in advance. Then I start flipping through the channels to make sure everything is kosher. Everything is looking good until I stumble on a few HD channels...some of them work and some of them don't. At this point I'm thinking "didn't I read somewhere that the Jensen shouldn't affect HD channels...???.” In the end, though, they didn’t work and I knew the Jensen wasn't going to be a viable solution.

So now I'm thinking 'great, I'm going to have to deal with the cable company to try to get them to fix the problem and that is going to result in me getting so frustrated and stressed out that I'll probably have an aneurism .' But then I remember that the Monster 3500 that I picked up for nothing after a mail in rebate from Buy.com claims to 'clean/filter' coax. So I plug the cable in there, run a line to the cable box and voila, no hum - and the HD channels work.

I couldn't believe it. I haven't had too much time to play around with everything to make sure that the internet, phone (both of which go through my cable company) and cable channels are all working correctly, but first signs are promising.

So, if you're trying to get rid of hum in your system and you don't want to run the risk of losing some of your HD channels, then you might want to think about running your coax through a monster power center.

I have no hum in my system with the Monster 3250 amp and Monster 5100 power center.

Has anyone arrived at a theory why the new 7500/7700 amps seem so sensitive to hum?

I had the problem, too, initially with the 7700 and not with the 755. Others have noted it, too. Steve said something about "differential amp" or "differential circuits"

I have no clue what that means. Perhaps GONK has some "input"

I'm not sure, either. From the posts I've seen here, it seems to only be particularly sensitive to ground loops when in unbalanced mode - the balanced mode (which makes full use of the differential design) is apparently the opposite extreme: not particularly sensitive to ground loops at all. I've suspected that the balanced mode is able to basically reject the ground loop hum (dealing with background noise and other electrical "harsh environmental" factors is what balanced cabling was designed for, after all). The unbalanced inputs would be their nature have to use only half of the signal path for at least the initial input section, and would then not benefit from that - making it more like a typical unbalanced design. As for why it appears more sensitive than other designs in that mode, you'd need to talk to somebody with a lot more expertise in electrical circuit design than me (my "circuit designs" are full of water or air - sometimes even hot air - not electrons).

My thoughts are the same regarding that balanced should reject. It was in the balanced mode that I had the hum. None in unbalanced.

Go figure