With the PA4X, I generally used the Automatic mic processor, so it pretty much maintains the right e.q., level, gate and compressor. Works like a charm.
The overall E.Q. of the keyboard does affect it too though. The MAXX settings are wonderful, but you have to experiment some to get the right sound for your speakers and venue.
With the S970 I'm using an external T.C. Helicon unit for several reasons. It sounds better than the internal Yamaha processor/harmonizer, it gives me an extra foot pedal setting for the keyboard because it has its own pedal, and it maintains a steady volume level. It also processes the voice automatically.
I have a spare T.C. Helicon Harmony M if anyone needs it. btw.