I would concentrate on getting as much room interior volume as possible, especially for a live/drums room.
The flutter-free ambiance that you can possibly gain by using angled walls, can be easily obtained by some absorption/diffusion panels put on the walls on a non-angled room, and so saving precious space.
That is of course if your original walls are not already splayed by construction, in which case you will actually loose room volume trying to make them parallel
