I just happen to have a Marshall slant cab loaded with G12-65's here, and it has a particle-board back, so I built a BB plywood back and ran small-signal tests on both of them. I'm not sure what the results really are, maybe you can figure it out. The green line is the BB plywood, the purple is the original back. The test was run with the Dayton Audio Test System, which measures impedance, not the actual sound level coming out of the cab, so that kind of complicates things, but I thought it was interesting anyway. To me, it sort of looks like the BB plywood enhanced the bass and cut the lower mids a bit, which correlates with what I and the owner are hearing; a bit more definition, and a little stronger bass. Still, it's not a huge difference, which makes sense, as both backs are under tension from the center brace. I inadvertently cut off the units on the X axis, but it's Hz, and the resonant peak is just above 100 Hz in both cases, so you can kind of go from there.