If you're going to upgrade to a 5.1 or 6.1, then you should go with two speakers and a sub. The sub will be absolutely necessary when watching 5.1 movies.

As for music, if you love anything with deep, pounding bass, then go with a sub as well. The M80's on their own will not give you the bass that an SVS will.

Also, a sub is self powered. If you want to power the M80's, you need to buy a very powerful amp capable of powering a 4 ohm load at a very high volume.

Stick with M22's plus SVS sub.