Snarl,
I have the 3805 with the 770 amp.
I have not had the opportunity to do an A/B with and with out the amp for anything other than 2 channel music.
I will say, I did not hear all that much of a difference in the mids and highs, but bass has definitely improved. It seems to be much more pronounced with the added amp.
I also expect the 770 to be an improvement over the 3805 when running more than just two speakers like in 5.1 or 6.1 surround.
Once I have my theater complete, an A/B comparison will be one of the first things I play with and I will post my findings in the 770 forum.

Craig