Scott: Hmmm... let's qualify my answer: I don't sing ! So my mic, which is for announcing only, is hooked up to the mic input of the 9000 Pro and then the sum stereo signal from both the keyboard and mic goes into one channel of the KP-200s. The amp has two stereo channels (each channel has it's own pair of L/R inputs) and Channel #2 controls a lo-z mic input (on the back of the amp) so it can be used as a dedicated vocal channel. Each channel also has it's own stereo enhancer control, which is a knob from 0-10, so you can turn the effect completely off and just have normal stereo - you'll find normal stereo to be a little dull in comparison though.

I don't think the enhancer will have any effect on your main vocal but it probably will affect the sound of your harmonized backing vocals if they are spread in a stereo field (it might make them sound better!). The enhancer doesn't do anything with mono sources, only stereo ones.

BTW: earlier I said that the stereo enhancer reversed the phase of the speakers - to be more precise, it changes the phase of the stereo SIGNAL, not the speakers themselves.