Gary, I do sing on my clinics/demos...and, I use the harmonizer...I use a vintage Electrovoice RE-15 on the Tyros4, and on the PSR-S910 (and previously on S900/3000/2000/9000/8000) a very cheap Yamaha mic that came with a vocal pack that was available back when the PSR-8000 was introduced.

I have had no issues with either mic...no feedback, or warbling sounds etc...just good clean vocal sound with and without harmony.

Of course, having the right settings is very important.

Ian