I recommend buying F. Alton Everest's "Sound Studio Construction on a Budget" (covers all kinds of rooms) or the 4th edition of his "Master Handbook ogf Acoustics". You'll see that he advocates diffusion rather than absorption for front and back walls. RPG's Skyline diffusor is a favorite of his. I have a DIY diffusor based on the same theory, if you can saw 2" Styrofoam into strips, then short pieces. I have Skylines on my back walls and my DIY's on the front, and the room remains lively enough, not overdamped (though Styrofoam does some absorbing too). I think you certainly can overdamp a front or back wall, or a side wall too, come to that. If you can try diffusion, I'm sure you'll like it. Please get in touch if you should want DIY plans.
