With the gear you have listed, and the fact that you have stated you will buy the 7700, I would recommend the 990 (run it in XLR (balanced) mode and see what you think. With the 990's 30 day return policy, it is hard to resist. I've had many HE processors thru my system (the last being the Krell HTS) and I chose the 990 because of it's features, SOTA processor, and off course, the price! While all the format wars continue, it is absolutely ridiculous to spend "many" thousands of dollars on a processor that may be obsolete in a few months or a years time IMO. There have been some reviews of late stated that there is "little" of no discernable difference between the AVM20 and the 990.
That said, everything is system and room dependent, so it is ultimately up to your ears to decide. Granted, if I could afford a Krell 7.1, that is what I would own - but would it be 5 times better (MSRP) - I don't think so.