I didn't audition the Denon when shopping for a receiver for two reasons: only one digital coaxial input and a fixed crossover of 80Hz. But, I must say, I've heard and read nothing but good things about the Denon's.
As far as Marantz, I was seriously thinking about buying a refurbed SR-8200 from www.ac4l.com. For $999 I think it's a steal. Personally, I wouldn't go any lower in the Marantz line than the 8200, just MHO.
I'm the owner of an h/k AVR520 (couldn't you tell?!) and all in all I'm happy with it. It finally came down to the h/k and the Onkyo 797 and the h/k sounded WAY better to me. The sound quality of the h/k through my M40's in "pure" 2-channel stereo is beautiful.
You should also keep an eye out for NAD's replacement for the T751, their new T752. It's not out yet, at least I don't think so, but it's specs look real good.
Good luck and happy hunting!

Big L