If you are handy and want to DIY some acoustic panels here is an excellent video:
IMO if you have a limited budget for listening room sound treatment you will be better off spending your funds on in-room treatments (absorbers, diffusers, bass traps) than on extra layers of drywall and insulation. I've heard several standard construction listening rooms that sound great, including mine, and if you sell your house someday you will not not recover the audio related construction expense.