I'll give you a little more info. I'm in the process of upgrading my console which I'm buying with a custom desk with built in patchbays. So yesterday I went to my studio and basically unhooked EVERYTHING (oh man I was in shock) to make way for the new goodies... ever notice how it only takes less than 20 minutes to have your studio in shambles.
So I figured that since my studio was all wide open I should get in there and do some more sound treatment as well. I'm actually very happy with my back wall. I have one DIY wall unit behind me (from ceiling to floor) in the middle to help with my imaging. It's about 10" deep with 6" of rigid rockwool with a 4" air gap. I also have two small bass traps in the back corners (these probably don't absorb that low).
At the moment, I'm most concerned with the wall behind my monitors, the one I face. But here's some good news. That concrete wall has actually already been framed with 2x4s 16" oc as Blue Bear mentioned earlier. I didn't realize that until I ripped a piece off today. So I figured what the hell, and completely gutted that wall. But at least I now have studs to work with on at least one wall.
I have a ton of literature from John S. an others that I've been collecting for some time. So I'm going to put something together for that "front" wall and I'm re-post here. Please check back because I love all the input.
Thanks.