DIY Room Acoustics

I've now done about every tweak I can think of to my system, so I'm thinking about low cost Acoustic techniques. I have done some things but would like to hear your ideas and tricks you've had success with. My room is 20'x 14'x 8' and has a wood floor with an 15' x 10' wool rug in the middle. One side wall is a large window with a curtain, the other side wall is floor to ceiling book shelves full of books. The front wall (behind the speakers) is a fireplace, and the back wall (behind me) has french doors with a hutch in each corner. The sound (of the room, without music)in the middle and front of the room is fairly quite, on the dead side. The area behind my listening position, between me and the french doors is quite lively, almost bright. My listening position is somewhere in between. I have very good focus within the sound stage both wide and deep. The sound stage tends to extend a couple of feet beyond the speakers. The treble has at times been a very slight bit bright, but I've controlled it with furniture re-positioning, set at a angle to the wall. The bass has had extremely good focus at times, but right now is just a bit murky. All in all the sound is very natural and if I could just get that little bit of base focus back it would be the best in all categories that I've ever had. I do not have any "room acoustics products" incorporated and would like to stay away from them if possible. Cost and aesthetics are both strong constraints for me. Thank-you for your great input on my other questions, I look forward to both your facts and your voodoo, it's all good here! J.D.