SR,
Two sheets of 3/4" heavy density or even MDF can be made to simulate a pretty good wall. The sheets can be added to the concrete walls in a tilted fashion to make the wall non-parallel ( you can do that in both horizontal and vertical if you have the room). Be sure to use 16" centers or less, and screw and glue everything together and the wall for most frequencies will be fine. Put the second layer on 90 degrees out from the first and insure that no joints in the first layer line up with joints in the second. The screw and glue technique will make the wall nearly impossible to remove but it won't vibrate or or interact with itself and it won't provide sympathetic vibration. The surface is smooth and can be covered in any fashion you want. Carpet will smooth out the high frequencies above 2kHz or you can stagger some live and dead material to smooth out the response but still make it live enough to feel comfortable when performing. Most acoustic musicians prefer some wood in the room to warm up the sound. Many studios use a wood floor but wood wall treatments also can help. Check out the RPG line of diffusor products here
http://www.rpginc.com/. I have used the Studio in a Box products with surprising results. The QRD diffusors and Omniffusor are also great products but are a little more pricey. You can even mount them on a movable partition (with casters) and have sound control and instrument isolation in one device.