inexpensive room treatment options

You have to be REAL careful when doing ANY type of absorption. Due to most "absorbers" not absorbing the entire frequency range in a linear fashion, they can actually do more harm than good.

Egg crates, egg crate foam, drapes, etc... will all skew the tonal balance due to the fact that they will do little to nothing for low frequencies while playing BIG games with mids and treble. As such, this can be beneficial for an overtly "hot" or "bright" system but can now swing the tonal balance too far towards the "warmth" side. This is not only true for "homebrew" designs, but also of "commercial" designs.

For more info on absorption rates and how it can affect tonal balance, take a look at some of the frequency response charts that Audio Advisor has for some of the Pro-foam and ASC stuff.

For well thought out and tested "homebrew" absorbers and bass traps, try looking at Jon Risch's site. He is both an EE and works with acoustics on a daily basis. Sean
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http://members.nbci.com/Jon_Risch/index.htm

Depending on your workshop capabilities, I'd recommend my very own DIY imitations of RPG diffusors. Please email for my instructions for making them if you could consider this: you'd need a table saw or some other good way of cutting Styrofoam cleanly and neatly, and a work table, and THAT'S ALL! I have 23 square feet of RPG-style diffusors on my front wall, three of them added one at a time, and each one made a major improvement in the sound. My electrostatics have a strong back wave, maybe you don't have that, but the middle of the rear wall is an important surface to treat, and absorption isn't the best way to go.

I agree with both Sean and Tom in regard to using diffusers on the back wall and withdraw the canvas painting suggestion (unless that is all that you can do). The paintings do however work at the first reflection points on the side walls and offer superior sound (to bare walls) in my setup. When I went back in time (in my mind) I recalled that the addition of either painting on the front wall degraded the sound somewhat (even with a cloth cover). The reason that the painting is where it is right now is because it is the only place that we could find space to hang it. I hope that this is not taken to be sacrilegious, but I enjoy the painting as much as I do the Hi-fi setup.

I find paintings to be terrible at the reflection points. Canvas may be good at absorption, but oil paint on canvas is just as bad as a bare wall. Maybe you are into modern minimalist art and are staring at a blank unpainted canvas with just a signature in the corner? Said with my best euro bohemian coice "It is the artists interpretation of the difference between audio cables or maybe the effect of a PowerSnake Hydra on the background."

Damn, I screwed up my attempt at humor once again. That should be "voice".