how can I make the back wall transparent?

After corresponding with a couple of acoustic treatment purveyors, I have despaired of finding any aesthetically acceptable rear wall treatment. Everything recommended is always some four inch or greater thickness monstrosity that looks like someone's hung a cloth covered shipping box on the wall. I have the same situation you do and would love to find some aesthetically acceptable way of reducing the rear wall's audible presence. Closest thing I've found that looks like it might have merit is the ASC Matrix panel. I've not found any feedback or reviews on it however.
http://www.asc-hifi.com/matrix-panel.htm

If you want transparent I'd go for a window :-)

Joking aside, In my room I treated the wall behind the speakers with RPG diffusers works like a charm. There are several other options available at lesser cost, if your handy you could even make your own.

Asmuck has a system page built here on Audiogon, where he utilize some of the same ideas- his room is among the best I've heard.

Best of luck

Peter

Peter, the acoustic designers I consulted seemed to feel that unless one uses massive amounts of bass trapping behind the listening position, one can't effectively ameliorate the total adverse influence of the rear wall. Out of curiosity, how close is your listening position to the rear wall? I've wondered how much of the problem can be ameliorated by diffusion, but I keep getting told there needs to be more distance between the diffusers and my listening position for them to be effective (my head's probably 18-24 inches or so from the rear wall.)

I think you are making matters worse by confusing what wall the OP is addressing. He really means the front wall (which is the wall behind his front speakers) People on this forum often call the walls by the wrong name. The front wall faces the listener and the rear wall or back wall is behind the listener.

If I am mistaken, I apologize. Maybe the OP can clarify, that way you are all on the same page.

Google GIK Acoustics seems you need some large bass traps behind the listening position. Give them a call I believe they can solve your problem. Good Luck