The problem is most definitely noticeable in a mono rig, moreso than stereo! If left and right channels are out of phase with each other and coming out of separate speakers at least it will sound ok in some parts of the room, but if you only have one channel, and you mix two out of phase signals together they cancel each other out before they even get to the speaker.
I think the EHX LPB-1 should do the trick and is pretty cheap. A guitarist friend of mine suffered the same issue running this pedal through the FX loop of his amp which was blended in parallel, so it must be a phase flipper.