Well I'm currently using a Korg PA1x Pro Elite and 97% of the time, there are no Mono Summing problems. I say 97% because the 3% of problems I do have are simply times when I'm playing a Midi Sequence and trigger another (on the other onboard sequencer) and fleetingly the sound gets noticeably thinner and "phasier" (it's a word, look it up ) and yet the mono sound is great overall.

I'd be surprised if the PAx2 isn't just as good, if not better.