Well now that there is at least one post with something other that a keyboard amp.....
I have a TubeWorks 300 watt Bass Amp with a Tubeworks 4x10 speaker cabinet. 2x15 does not compare to a 4x10 cabinet. A lot more punch from the 4x10's. This put the Roland KC500 to shame. However, it costs a lot more than a Keyboard Amp. There are other possibilities. One of which are powered speaker/monitors. The JBL Eon series are quite good. Check around. The advantage that a Keyboard Amp has is that you have more inputs, line out capability, built in reverb and eq on some models. Some people use a speaker, an amp and a mixer. The Keyboard Amps just combine all three.