I know that Boo and some others are laboring over which direction to go concerning a p.a. system.
Before making a decision in this area, you must consider monitors.
Doing a single, I have tried various ways to monitor my performance. Many times I have simply plugged a couple of hotspots into the back of the mains. Not optimum, but you can hear yourself very well. They have excellent high-end response and you can hear the bass, which is basically non-directional, from the back of the mains pretty well.
If your keyboard has built-in speakers, they can be used as your monitors in low-level situations, but I find they aren't loud enough for many jobs. If your keyboard doesn't have speakers, and you plan to use self-powered mains, you must certainly address this area as part of your decision.
You can NOT just plug extra speakers into the back of them. Your monitors must have their own power source.
Yes, you can set up in front of the mains, but at any high volume this will cause feedback, particularly if there is an extra mic attached.
I have lately experimented some with using headphones (the earbud type are quite unobtrusive). This does limit your mobility a little, but it sounds like you're in a recording studio. Needless to say, transportation, cost and set-up is minimal.
Ever notice when somebody says "needless to say" he says it anyway?
DonM