I really like the J. Mascis acoustic album. I assume we are referring to the CBGB one. Anyway, my M discs cover:
M83
Magic Numbers
Stephen Malkmus
Magnetic Fields
Mansun
Roots Manuva
J. Mascis
Laura Marling
Massive Attack
Maximo Park
Mazzy Star
MC5
Melt Yourself Down
Mercury Rev
Metronomy
MGMT
Minor Victories
Mitski
Mission of Burma
Moby
M…