Anything that stiffens the walls. The purpose of bracing is not strength, but removing the tendency of the cabinet to carry sound waves in an organized manner. I might be a little suspicious of bracing that forces too much pressure on the sides, as I would feel that eventually it would fail at the points of pressure. I could be very wrong, but I wouldn't build one that way. I would almost think that that might create its own resonance problem.
Gone are the days we stopped to decide where we should go, we just ride...