AN, sure, a sub such as the 20-31 would definitely help, but just adding another pair of speakers, such as the M3s, doesn't appear to be something that would be worthwhile.
Your "What to add?" clearly referred to bass effects, but you may be under a misconception as to surround sound. The benefits certainly don't apply only, or even primarily, to movies. My music to movie ratio would probably be on the order of 99 to 1, but I always use processing such as DPLII to listen to 2-channel music sources in surround sound, for a closer approach to a realistic home audio experience. If you add speakers, you should consider a surround setup using QSs.
