I used to use an old powered mixer for my many keyboards. I find that the Korg amps with a lot of Inputs are nice, but they get muddy when you go for multiple items at high volumes.
I would check out the soundcraft GigRac 1000ST. If you can spend $1500, you could get a very nice setup. I would recommend getting a stereo unit and two 15" Pa speakers with horns. Maybe a small powered sub from the monitor mix.