don't know about the HK, but I've had my Denon 3806 for a year now, and I still very happy w/ it. It does a wonderful job and is flexible enough to suit a lot of needs, and the remote I don't mind either, some people don't like it. I didn't care for the HK's last year, but again, I don't know about the 645 specifically, I haven't looked at them in a year, so I don't remember which model #'s they were. Last year though, I was getting more receiver for the money w/ the Denon, don't know what the current HK line offers though.
my 2 cents