The easiest way to improve room acoustics is to start with bass traps in the corners. I built my own using 4" Roxul 2'x8' mineral wool boards enclosed in 1x4" pine boards and covered with speaker cloth. One of these in each corner does wonders for reducing peaks and raising nulls as well as lessening resonances and room echo. The materials cost around $50 and the results were immediately noticeable as tighter bass and clearer mid range.
After living with that treatment for a while you may want to experiment with front and rear wall treatments. Heavy curtains also work well.
