how can I make the back wall transparent?

Have u looked thru the virtual systems for ideas?
In my case; room is 12 x 16 feet, speakers are 3' from wall; toe-in made a great increase in soundstage plus I'm using an equilateral triangle method. As in many of the virtual systems, I used bass traps in corners and 2 vertical DIY vertical panels on wall behind speakers; that wall then disappeared and became like a concert hall.
BUT, the panels are ugly...2 inch foam, mounted on boards that I can stow away when I have visitors. Obviously I'm not married.

"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."
I am using bass trapping panels and it works for me; in a small room you don't need massive amounts.

Williewonka, sorry to have misinterpreted your post and hijacked your thread. Needfreestuff, GIK was one of the firms I consulted and they did indeed recommend fat bass traps on the wall immediately behind my heaad. Problem is, they are visually intrusive and incompatible in a well designed domestic environment IMO. I have bass trapping in the upper room boundaries in the "plant shelving" recesses of my room and it helps a great deal. It works basically like the bass trapping Peter describes. At least I don't have nasty room response problems, the space measures pretty good. I have a medium size 18.5' x 14'room with 11' ceilings.

Bass reflections off the wall behind the speakers does not affect perceived image depth. It's the critical midrange where you want to get rid of some reflections.

It all depends on what you call "pleasing" as far as looks.

A few panels of strategically placed Sonex will help immensely.

Some companies make some fabric covered shallow boxes filled with sound absorbent material (fiberglass....) that would do the trick.

Nice Persian carpets, that you so often see in pics of listening rooms, help.

Your photo or a stock photo on a panel.
http://www.acoustimac.com/acoustic-panels/acousticart-panels/

I agree, Tom, I think the op is talking about the front wall.

I'd recommend treating the room with bass traps / absorption panels and diffusors. I'm new to room treatments as I just hung my first four panels (gik 244 broad band traps) but I'm totally sold on room treatment now. I think you can gain what your looking for by treating the first reflection points and going from there!
Good Luck!