Owens-Corning 503 is pretty much the standard stuff used for acoustics. Other names are rockwool, fiber board, and others I'm forgetting, but it's the stuff used to insulate ductwork in buildings. It doesn't have to be 503, any stuff you find that's of a comparable density will work fine. The denser the better really, especially for bass frequencies.
Put on some music in the room at a volume comparable to what you play at, and walk around and listen to how the sound changes in different parts of the room. Stand in a corner, and more than likely you'll find that it sounds a little bass heavy, maybe even muddy. Low frequencies love to build up in corners. Bass traps are your friend. It doesn't have to be anything fancy, even an old couch standing on end can make a huge difference (if you have the room).
When it comes to acoustics, (almost) anything is better than nothing. The comforters will help, but for $100 you could do a lot better.