Another reason to go with M2 and sub will be the proximity to the wall. Too close to wall might bump the bass of the M3 even more. M2 won't have that problem.

Since you already have M3 I would go for M2, just for the fun in the variety. If the M2 doesn't work out, send them back and try again.
M3 and Manley Stingray
M80 and Pioneer SC 55