Ichigo,
"Flat" can have several meanings. In terms of dimension, the M22s are anything but "flat." With good recordings, they can project a wonderfully 3D soundstage.
"Flat" can also mean "linear", as in a speaker having a flat, smooth frequency response, and that is a compliment. The M22 is very linear and will be brutally revealing of poor CD recordings, and will sound glorious with good recordings.
The M3, as has been pointed out, is not quite as linear or "flat" as the M22, and has a bass hump (boost) that's quite appealing on some recordings because it makes the M3 seem to have more bass output than more linear speakers like the M22 or M2. The M3 is also a bit less detailed in the midrange, so it's more tolerant of lousy recording quality than the M22. My preference is for the M22 with its sparkling detail and wonderful soundstage, but the M3 is a very nice speaker with legions of fans.
Regards,
Alan
