how can I make the back wall transparent?

Williewonka, thats some really nice effort you have been putting in.
Give the ASI Liveline resonators a try. They essentially do exactly what you are trying to achieve, they make the room disappear! And they are extremely small in size, like a match box each. The big deal about them is, their effect is like the walls have disappeared but the musical energy which should be present in a small room remains with great open-ness. They not only improve focus and soundstaging but also greatly help in improving the immediacy, dynamics and timbral accuracy. Every wall (and for that matter every item) in the room would be adding or taking away something from the tone, the ASI makes the wall disappear in the most graceful sense, as in tonality and frequency response is also corrected. Talk to Frank Tchang and give a basic set of resonators a try if possible. Your wife wont even know you have added something to the room, they are that small.

Achieving so much benefit from treatment in the upper 15"-24' of the room is what surprised me the most.

That's the way Peter(Pbnaudio) treated his rear wall; absorption along the entire upper wall. It's even more effective in his case since he has space between his listening position and the wall.

I'd like to do that, but my setup is in a living room.

Lowrider57 - my setup is also in the living room, so the 24"× 60" piece of white vinyl currently supported by two stands is a bit of an eyesore right now.

However, I've just ordered a 92" × 78" vinyl roller blind ($215) that will fit behind the drapes and is almost invisible when retracted. High on the WAF scale :-)

Perhaps you could mount a similar blind with a white valance to "hide" it
-after all, it will be up against the ceiling (retracted) - no one looks up there :-)

If it's a plain wall you could put an outrageous work of art under it to divert peoples attention?

Possibilities are endless :-)

$215 is not the cheapest tweak I've implemented, but it has been the most effective to date.

Even my wife thinks the system now sounds amazing :-)

Cheers

Pani, many thanks for the tip.

Everything you said in your post about energy, sound staging etc... - basically all the benefits you mentioned - is exactly what I am now experiencing.

However, I do have a couple of albums that seem to be a little bass heavy - so once I've determined that it is due to room acoustics and not the actual recording I'll definitely give them a try.

One album - Eric Clapton's "Unplugged" had some seriously bloated low frequency foot tapping going on in my room, which has now subsided to a more realistic level, but it could still do with some more "fine tuning" to get it perfect.

Cheers

Glad to hear the happy ending.

So substitute for determination and trial and error over time when it comes to managing room acoustics. So many ways to skin the cat.