Ian,

I haven't used a mixer in more than 10 years. Every external vocal processor I've owned has phantom power, therefore you can merely plug the output into the keyboard's Line-In and you're ready to go. Additionally, you can still utilize some of the great vocal effects in the keyboard, reverb, delay, etc..., all in conjunction with the outboard vocal harmonizer.

As for using the onboard speakers, I wouldn't recommend it. Too many problems with feedback and looping because of the close proximity to the mic. Dead-plug the speakers and use a good PA system and those problems are non-existent.

Cheers,

Gary