I've experimented a lot with switching my receiver back and forth from "direct stereo" (only M60's) to "straight" (M60's and my SVS sub) while listening to music, and I'm always impressed by how capable the M60's are all by themselves. Sure, there's a bit more low bass with the sub, but for stereo music I don't think I'd feel at all deprived running M60's without a sub.
For DVD movies, though, the sub is essential for those low effects.
Haven't heard the M2's, so I can't say what they'd be like with a sub.
