I have the 507 and don't have the humm and whine problem. Thus far, I don't have major complaints, but I'm just getting the sucker broken-in. I read tons of reviews and listened to Rotel, NAD, Arcam, and Denon before making a decision on the 507. I purcahsed it from a local Tweeter, so I've 30 days to determine it's worth the $2900 I spent on it. I plan doing side-by-side comparisons with Rotel and Arcam for sound quality. The 507 is rich in features that the other may not have, but great sounds beats out a second zone that will probably be never used. I'm been searching for "professional reviews" on the 507, but havent been able to locate any, so I've based my decision to purchase on 307 reviews. The big difference is additional sound formats. The amps and sound are the same for two-channel, this comes from B&K Customer Support.
I've been searching for a forum dedicated to 507 users, but have found one. Maybe, I'm not looking hard enough. At any rate, good luck in your search!