Ian, the phasing issue shouldn't change whether you use the mono output on the PSR, or send left and right to an amp, and mix it to mono there. The issue is, the samples themselves have phase issues, and unless listened to in stereo, no matter HOW you collapse them to mono, they will still have problems.

The only thing that perhaps makes a perceived (but not actual) difference is the overall 'sound' of the rig being played through. Phasing problems are usually perceived on higher frequencies, so any sound system that accentuates highs (like Bose) or decreases lows and low mids (like many smaller rigs) will have the effect of making these problems more noticeable.

But WHERE you mix an out of phase signal down to mono should have no real difference to the final sound. Only the speakers will change that...