Jerry, welcome. In my view the term "3.1 HT system" is something of an oxymoron, since surround sound is essential to the home theater experience, including 2-channel music. Speaker positioning is often "problematic", but the problems can almost always be solved. If you'd give some details, we may be able to offer some helpful suggestions.
To match your excellent M22s, a third M22 used as a vertical center would be best, or an M2 would also work very well. This is another application where positioning problems can usually be overcome.
The 591 is a fine receiver. You should consider getting it or a similar model as a factory refurbished unit from a source such as Dakmart or eCost. These are likely to be at least as reliable as a brand-new unit, but with a significant price saving.
