I agree with Bren's general concept. It is a common application in other basement HT rooms that have been built with thought. The more layers you have between two rooms/areas, the more sound vibration will be dampened. Use 2 layers of drywall, plus insulation, the more dense the better.
The best solution of course is to build a room within a room but often that is too costly an option.
Our new basement HT has ceiling batting, plus the walls all around are filled with a common Revy sound insulation material (non-formaldehyde production). I skipped on the second drywall layer as i did not want to make the room "too" soundproof, just sound controlled. The results have been excellent.
