If you did correctly what you described it should work. There should be no differences between monitor cab off and cabinet off. Furthermore in release 5.3.1.13085 there is none.
I checked this by reamping the same riff over and over with different settings and inverting the phase in one track. If those tracks are identical you hear nothing. That's the case, as you can "see" in the picture below. So either you've got a defective unit or defective ears
...or they've changed something in the public betas. Please give us some sound samples and tell us which firmware version you've got.
Changing settings like low shift in the cabinet does make a difference even if the cabinet is off. But that's known and not part of your problem as it appears in both cases (monitor cab off and cabinet off).