Volume is displaced by the braces, but the difference between 10 linear feet of 1" x 2" stock and the same length of 3/4" x 1 1/2" stock is only 100 cubic inches. That's a very small percentage of the 9000+ cubic inches displacement of the tower two π loudspeaker cabinet.

So brace well; Don't fill the cabinet up with braces, but don't be concerned by fractional differences either. You'll find that the tolerances are wide enough that bracing volume is insignificant.