I have 5.1 fronts, c3 center and 4.1 rears and was powering them with an onkyo 804. It sounded great when I frist got it...much better than the Aperion system I was replacing. However, over time I felt that the highs were not airy enough and the mids were not as punchy. So I added am emotiva lpa1. Right now I have the 5.1s bi-amped and the c3 being run with the emotiva. The difference is quite extraordinary. Suddenly the highs are much airier, the mids are much clearer, I can hear more detail in the instruments and the soundstage has improved. In each case, the differences are quite subtle actually, but in totality it sounds much more melodious.
I know that passive bi-amping is not supposed to be that useful, but it does seem to give just that little bit of difference which makes the sound that much cleaner. I tried it first tried using the new emotiva without bi-amping and did not notice as much of a difference but now it sounds really nice.