[Otto]

Hey there,

What's your crossover point? I'd expect to see the most change around that area.

Is there perhaps a "bypass" switch for the phase control?

I don't have a phase control myself, but I can invert phase (180 degree shift), and I measure a difference. I would expect that you should at least see some minor changes. When using REW, you can overlay all your measurements on one set of axes; it's much easier to see differences that way. Just select "All Measured" on the tabs across the top of the measurement display.