As you have found, there is not a way on the switcher itself to directly reverse the phase of the outputs. This problem can easily be resolved by reversing the speaker connections on the offending amp(s) cabinets. Because your amps appear to basically be similar in design, it is possible that one or more of the speaker cabinets are already wired out of phase, relative to each other, to begin with. It isn't as simple as flipping a switch, but once resolved at the source of the problem it will always be correct
I hope that helps!