My real concern is timber matching all speakers for best sound. I know the VP180 plays well with the M80's but which of the bookshelfs are best suited in this combination?
IMO the M80s, M22s, M2s and QS8s are as perfect a sonic match as you will find with any system of speakers. I can’t comment on other Axiom offerings since I haven’t heard them but based on the driver complement and comments from other assume that the VP180 is also a perfect match.
My room is 21 feet deep with the front of my M80s 4 feet off the front wall and my seating 12 feet from the mains, about 11.5 from the center which give me about 5-6 feet behind me. I’ve tried M22s, M2s and QS8s as rear speakers in this room and found no difference between the M22s and M2s. IMO the QS8s are marginally better as rears even at 5 feet because the way the spread out the sound helps them blend better with the QS8 surrounds. So if money is no object I would go with QS8 rears, however if you want to switch out to bookshelf rears I’d go with M2s.