I do piano gigs with my roland rd-700, and get great sound with a pair of Mackie SRM-450's. These have loads of very clean power, and simulate, in a large room, the sound you can get on your headphones. Their weakness is probably the low bass--they have quite a bit of bass but it doesn't extend quite as low as an acoustic grand piano. Still I find them sufficient. They are easy to carry around, can be positioned in multiple ways depending on your setup. With two, it is easy to use one as a monitor and the other facing out into the room. If you want to go smaller for a gig, you can get by with one. You probably need quarter-inch to XLR adapter cables. If you also want to use a mic or another keyboard, you need to add a small mixer as the Mackie's only have one input. At that point you basically have a PA system.
I have a very good hi-fi stereo setup at home, and when I play recorded music through the Mackie's the sound quality compares well to the stereo (and they were much less expensive). Most dedicated keyboard amps, in my opinion, are not worth considering. They just don't sound that good, are heavy and clunky, and not as flexible.