Tech, $899 is an an excellent buy on the 876, which is a very full-featured(e.g., Audyssey MultEQ XT with Dynamic EQ, Reon video processor, etc.)receiver with plenty of power, including at 4 ohms. It should work very well with your M80s, which are a bit higher than average in sensitivity and require less power than many other speakers.
As to having a separate center speaker, if you're presently satisfied with the phantom center that you're using, then it certainly isn't necessary, but if you can temporarily use some speaker as a center as an experiment you can possibly judge if you like it even better that way. The usual downside of the phantom center is that the image of dialog or other material in the center channel will move to the side of a listener not in the center(but not farther out than the main on that side). Another point is that a separate center speaker can have it's volume level bumped up a bit to help the intelligibility of dialog in certain movies.
