Please dont! dont! DONT! use guitar or bass amps, they absolutely suck amplifying keys and thats simply because their not meant to do that, theres so much frequencies they miss...
You need something that covers a wide range of freqs, from deep lows, warm mids to (specially) crisp highs. You basically have 2 options: near field studio monitors, that may give you the best quality but less power (theyre designed to sound really good at a small place) or powered speakers that may give you more power but rather not same sound quality as studio monitor (but good enough quality, infinitely BETTER than guitar amps anyway...). 8" speakrs will be enough for deep bass on your lows.
You can google about this, it took me some weeks of reading and asking to find out and decide myself. If you wanna know I went for powered speakers, since I get a lot of live gigs so I use them for amps or monitoring.
Good luck!