With your ceiling height, the M-2s or M-3s would easily fit. With a slight customization at time of manufacture, the M-1s would fit. One of my customers has a similar ceiling height and wants M-1s if possible, so I have researched the possibilities. Standard M-1s are 80" tall as standard, which combined with 1 1/2" tiptoes, are probably too tall to fit, considering setup allowances.
As you go up in size with the speakers, the horizontal dispersion increases as well. The M-3s have 60 degree dispersion, the M-2s have 75 degrees, and the M-1s have 90 degrees. There are reasons for the differences, one being that the smaller speakers are typically used in smaller rooms and are intended to image well in those rooms. Sound Lab's research indicates the optimum dispersion angle to be 90 degrees for best reproduction of the reverberant sound field. Additionally, the big Sound Labs have a fuller tonal quality and richer harmonics than the smaller ones. In a small room the natural room reinforcement of the bass helps fill things in. As room size increases one might feel a need for subwoofers, which are difficult at best to integrate with the main speakers. My recommendation is to go with the larger speakers if you can, as you can always turn down the bass on the backplates. You most definitely wouldn't need subs with the M-1s.
Rwd is correct, the vertical dispersion of the speaker is virtually nil, hence floor and ceiling reflections are of little concern.
Your room size of 15 by 20 feet will allow any of the models to work well. Listening nearfield is no problem, and you will enjoy it immensely.
Pass X600s are known to drive Sound Labs, although they may be overkill. The smaller Sound Labs are a bit less efficient than the larger ones, so I can't say how well the X250 would do with the M-3s. I imagine it would do fine with the M-1s and perhaps M-2s unless you like to play your music really loud on occasion. One of the neat things about Sound Labs is how great they sound at low to medium levels.
Good luck! If I can be of further assistance, please feel free to ask or call.
Brian Walsh
(847) 382-8433
