IMHO, using guitar and bass amps for keyboards is a bad idea unless there's a particular sound you're going after, and only a guitar amp will do. For example, I remember hearing that Jan Hammer played his Minimoog solos through a Marshall tube guitar amp to really get a screamin' lead sound.
Guitar and bass amps typically are not designed to reproduce the wide range of frequencies that keyboards can generate.
Roland makes a whole line of keyboard amps that can be paired up to run in stereo. I've got the 120-watt KC-350, and have used it at countless gigs. Pros: Loud, well made, small, self-contained, four stereo inputs, stereo link capability, three-band EQ, tape in, mic in, lots of low end. Cons: Heavy, expensive, too much bass at times (crank the EQ down), less than stellar for piano sounds.
Many keyboardists prefer to use powered PA speakers for live use. I recently purchased a
Behringer B212A for $229 and this thing CRANKS. 400 watts and it sounds great. I use it with a small, inexpensive keyboard mixer. It weighs less than the Roland KC-350 and provides much more accurate sound reproduction. It does not, however, provide the same low end that the Roland does.
Ultimately, I'll likely upgrade to something like the
Mackie SRM450.