The only time I use a pre-amp/mixer with my PSR-3000 is when I have other singers and guitarist working with me. Then I plug an Alesis 6 channel mixer directly into the keyboard's mic/line-in. This allows me to control individual volumes at my fingertips and control the overall volume with the master volume of the keyboard. This makes life a lot easier because when you need a volume increase to accommodate changes in conditions, you don't have problems with one person cranking up their volume well above another. Once the balance is set, the only volume that gets changes is the master keyboard volume. All of the vocal effects, reverb, delay, EQ, are built into the mixer and it only cost $99 U.S. dollars.
PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)