Offhand I don't think I could go wrong with either brands. The GFA7700 is a 175wpc brute, rated with All channels driven into 8 ohms. The 1075 is also a 5-channel power amp rated at only 120wpc at full bandwidth. From the tech specs alone I'd get the Adcom. Both amp are rated conservatively.
The Denon 3803 is a 7.1 receiver tauted as a 150wpc rated at only 1Khz into 6 ohms with a THD of 0.7%. This may be more realistically in the 120wpc if rated conservatively with all channels driven into 8ohms at full bandwidth. It would seem to me that upgrading to a conservative 120wpc Rotel may not give you a very noticeable volume or SPL change with the same volume setting using the Denon receiver as your pre/pro.
For this reason, I'd opt for the more meaty Adcom, based on specs alone that i've seen on the net. I would suggest you audition the two using the same speakers you have in mind and preferrably with a denon 3803 or something similar driving it. That should be more telling about the sonic quality you can expect when you bring either to your home. But if you can borrow them for an over-the-weekend demo at home, better.
Just curious though, are you sure you want to go 5.1 from 7.1? Or are you also planninng to get a stereo power amp to drive another pair of channels?
