I have a pair of M3 tis, a relatively recent Yamaha RX-396 Receiver and a room that's about 12'x18'.
What should I buy to improve this setup?
I was considering a sub, but I live in an apartment with neighbors below. Also, I'm not much of a fan of bass-heavy music, I'm just after fullness and overall sound.
What are you doing with the system? Music Movies?
If properly set up, a good sub dosn't give you heavy bass. I listened to the M22s with an EP350 and I really liked the bass for music. It was not heavy at all. After level matching you can always adjust the bass to your taste.
It's a system for music, primarily, though at some point I may join the rest of the world and connect a TV through it.
Definately add the sub. It will fill out the bottom end on the M3s.
Of course, you could always just go for the M80s...
Major, welcome. You should also consider adding a pair of QS surrounds. Even for music, surround sound adds significantly to the listening experience. Modes such as DPLII and Neo:6 can be applied to CDs and other 2-channel source materials to send the ambience which originally came from a direction other than the front to the surrounds where it belongs, and makes listening a little more realistic.
Welcome to the boards Major.
I would suggest adding floorstanding speakers as the mains and then using the M3s as surrounds. The fuller overall sound of a floorstander is much better, IMO, than any bookshelf even with a sub. I would recommend the M60 or M80s. You will also most likely find the detail from the M60/M80 easier to hear than from the M3's, at least it was for me.
My next upgrade selection would be to then go with adding a sub as the next best upgrade and then simply adding surrounds as the last upgrade to make.
No, I'd add the sub before changing out your mains.
I would recommend surrounds before the sub, everyone has their tastes I guess. The M3s put out decent bass, you won't know you're missing anything. If you get the QS series surrounds it really will make your system envelope you in movies.
I like Jakewash's Idea of upgrading your mains to floor standers and using your M3s as surrounds. A good use of your current resources.