Jack,
Welcome to the forum. Your situation is different from mine, in that you already have the M60s so, my information may be unrelated. I was looking to buy all new Axiom speakers, but could not decide between M60s and M80s, so I ordered both to test out. If you are not looking to consider getting M80's (or ordering to test and return for full refund < 30 days, then ignore this post. (note: you'd pay $60 U.S. for return freight)
If you are interested in reading my A/B tests of the 60s vs. 80s in a large room, check out this thread.
Dave's M60 / M80 A/B test (just my opinion) Go to page 3, 7/7, post # 255939 and Page 7, 4/11, post 256396.
My room is 3l'L x 23'W x ≈ 9" high (ht. is avg of vaulted ceiling). My room is half carpeted (the half that is the listening area. I do have to 8' sliding doors on the right wall, but it sounds like you may have much more glass than my space, as well as less sound absorbing furniture. IMO, your space would amplify the highs by reflecting them around more. I know wall hangings have been suggested, but my understanding is that you have more windows an not too much wall space. If drapes fit in to your decor likes, they would help a lot. Carpets on the hardwood floors at stated would help the sound a lot too. BUT, if you are not interested in adding lots of sound absorbing things, want to just use speakers that will accentuate the room's reflectiveness just a little less, then as mentioned a sub would be important toward that end. IMO, M80's are hands down a much better choice for a large room than M60's.
Especially a large and very reflective room like yours.
Good luck in finding a compromise that best fits your decor, desired budget and 'effort level' to attain a sound you can live with in your space.
