It's hard not to restart the Emotiva/Axiom discussion when answering your question... Bottom line, it works for some, not for others. There seems to be a greater chance of it not working with Emotiva than with others, even if the overall chance of it not working is still small. Given what I've read here on the topic, it would not be my first choice, but I have no first hand experience, just that of reading numerous accounts of "my Emotiva works, mine doesn't."
Re: 806 versus 906, it's up to you to decide whether the additional features justify the extra cost. Just focus on features and not on power as the small difference won't have any noticeable effect on sound. If you don't mind the features you give up going to the 806, that seems to be the better option between those two choices.
The last question I would ask is do you need separates... Like David said, it comes down to listening room size and how loud you listen. If you have a big room and like to absolutely jam (~100 dB, plus more for peaks), you're probably an ideal customer for separates. If you listen at moderate levels and want an amp that will just handle the peaks from there and do not have a huge room, the onboard amps of any of the receivers mentioned should be adequate.
A long post with no direct answers, sorry I can't be more definitive!