There's an acoustic kit I bought last time I was in England so I don't know if they're available here
called 'PrimAcoustics' . It basically consists of kits ranging in price from a coupla hundred to a coupla thousand pounds depending on the room size. They're preshaped acoustic foam 'blocks' that you attach to the wall with either a glue gun or if you wanna move em around use velcro. You don't cover the entire wall...just sorta place them in areas where sound is getting badly reflected. You can cut em to size too if ya want.
As soon as I put em up I noticed a huge difference in sound quality. All that high end metalic ping sound had disappeared and on the playback monitors, all the bass tightened right up.
Best thing is you can keep a small 6' X 3' kit in your car and put em up temporarily when you need em.