I haven't noticed a problem with my right front M3v2 in a closed bookshelf. I just measured, and the back of the M3 is 6 inches from the solid back of the bookshelf. My M3s are set to "small" and are crossed over at 80Hz, so they don't need to supply much mid-bass.

Greg
VP180, M80s, M22s, QS8(4), CSW S305s, EP500, Pioneer VSX-90
M2i, M3(2), Pio vsx-1020