Lots of good suggestions here. I incorporated many/most of them when I built a 38x28 addition on the end of our home. Concrete floor with extra foundation against earthquakes (it IS California!). Double offset stud walls using 2x6s, soundproofing, double door into room from remainder of house with one door constructed with internal sound deadening, etc. The only suggestion I would add that I did not see above is to avoid a flat ceiling (maybe I missed it); mine ranges between 8 and 10 feet peaking off center. Sonics are superb and my son's blues band uses it for practice and recording. Good luck.
