I have had the 8805 in my system. It has excellent bass and midbass fullness and strength. Excellent midrange and impact. The high frequencies are softened and rolled off somewhat. I don't know that you would notice this unless you did a side-by-side comparison with an Anthem or Krell Foundation.
The 880x has a main power supply that is twice as large as the 770x. 880x I think is a total of 40,000uf. The 770x is 20,000uf. Of course, the 880x has all the discrete analog output channels on individual boards (13 of them in the 8805). I would think the sound between 770x and 880x would be similar. The 880x would likely have stronger/fuller bass and midbass and an overall smoother sound.
I was very impressed with the 8805. The high frequency difference only became apparent when I hooked my Krell S1200U back into the mix.