Comparing the 755 and 770 is pretty easy - they are identical except for the number of amp channels provided. The individual amplifier channels used in both are the same, as is the chassis. There are some slight differences in the power supply, I believe, due to the extra channels in the 770. Sonically, everything I've seen indicates that they are indistinguishable from each other. If you think you'll go 7.1 and prefer to keep your amps in one box, the 770 is just what you need. If you think it may be a while (if ever), you can always elect to get the 755 and add a pair of M200's later.

There's some scattered info on the 790. It was originally reported on in this thread , with some good pictures of it provided here by psklenar. More recently, this thread had a couple of reports.