Hello Tdanalog- Whatever you may end up using: Type, "acoustic foam" into the searchbar on eBay. There are a number of products in there designed to tune your room against unwanted reflections and control resonances. Auralex panels and LENRD bass traps are great(the Maggies I'm using for mains are closer to their back wall than I'd like them), and will absorb loads of unwanted energy. You needn't cover the entire wall, but rather experiment with breaking up the area directly behind the speakers(as Newbee suggested: whether dipolar, bipolar, omni or front-firing). Also, being a narrow room, it would be best to control the reflections behind your listening position as reverberation effects will play havoc with freq. response and imaging. One nice thing about panel speakers(like the Quads) is they are pretty beamy and you don't need to worry as much about side wall/ceiling reflections with them. I had some Acoustat Model III's back in the day(early 80's), in a less than perfect listening room. Still got them to image/sound great using carpeting samples on the walls for treatment. Oh- on eBay there are a bunch of Auralex knock-offs that would be just as effective and won't break the bank. Those 2" X 24" X 24" pieces look like they'd be perfect for what you need, and the "bass traps" If you're close to the corners of the room. The stuff is easily hung(and moved around if necessary) with latex caulk. That way there's no permanent damage to your walls. Happy listening!
