Okay... a little info: Guitar amps are designed to amplify guitars. Keyboard amps are designed to amplify keyboards. What do I mean by such a seemingly stupid statement? Well a guitar's range is much smaller than a keyboards; they tend to play 4 octaves mostly. A keyboard, depending on the sounds it is emulating (let's say piano), uses 7 octaves, and overtones, and whatnot. Honestly, a bass amp would meet a keyboardist's requirements before a guitar amp would (indeed, a popular setup for some pros are two accugroove 112 bass cabinets and a QSC power amp).
Another thing, is guitar amps are as much a part of a guitar player's sound as the guitar. They tend to prefer tube amps to have some growl and overdrive to their sound. Overdriven piano would sound like crap. Overdrive, if you don't know, is similar to distortion, only instead of always loud, it is a warmer and subtler sound. It happens when the tubes of a guitar amp's tube amp warm up from playing. The louder they are pushed, the more distortion, or overdriven, they get. Not ideal for piano playing, though possibly for Wurlitzer...
Okay, so why do keyboard amps suck? Basically, for keyboard amplification, you want whatever is amplifying the sound to simply make it louder. You don't want the mids, highs, or lows, to be accentuated, you (typically) don't want overdrive. Basically you want what's referred to as a flat, clean sound. Not flat as in boring, but flat as in the amp does not change the quality of sound.
Okay, another thing is that a lot of samples on a keyboard were recorded in stereo, so they will sound better in stereo. Plug a keyboard into a keyboard amp (or guitar amp) mono (meaning, one plug into a single speaker) and compare that to the sound if you plug in your (decent quality) headphones. Yeah. Yikes.
So: you want something that's going to amplify all frequencies to match what you would hear acoustically. You also want stereo. Alright so generally keyboard amps have 2 speakers, a woofer (larger speaker, 10, 12, or 15 inches) and a tweeter (1 inch speaker designed for highs). For whatever reason, keyboard amps have a hard time amplifying the middle range of a keyboard. Lows can be fine, highs actually are tough too, they tend to be piercing, and the middle range tends to suffer. Three ranges (basically): low, mid, high. It's very difficult to get all three to sound good. You get lows and mids sounding good and the highs will be piercing, etc. Another thing is stereo. Most keyboard amps (excluding the Traynor K4 and the Motion Sound KP100s, KP200s, and the soon-to-be-released KP500s) are mono, because there's only one woofer and one tweeter. So if you want stereo you'd need to buy two. Keyboard amps tend to be heavy if they're powerful.
All this begs the question: what the heck do keyboard players use to amplify keyboards if keyboard amps suck? Well since we don't want any colour added to the sound from the amp (by that I refer mainly to overdrive), basically we want a PA system. Stereo, so two speakers. Possibly a mixer. Speaker size is something players think about. Okay, so there's a few different options on what to use. They differ on a few things, the first big one, is are you playing live or at home/in the studio?
First live sound. One option is powered speakers. http://www.musiciansfriend.com/navigation?q=jbl+eon&st= http://www.musiciansfriend.com/navigation?q=mackie+srm&st= http://www.zzounds.com/item--ELVSXA360
So you get two of those. If you get the JBL, you can stop there. If you get a Mackie or EV SXA360, you will have to buy more things, i.e. a small mixer (for EQ control and to convert your signal so you can plug into the speaker... more on that later perhaps). Speaker size depends on how much bass responds you need or want, and how loud you want, etc. Too many variables for this post, possibly down the line.
Another option for live use: Two (or one, you could run mono, same with powered speakers, nothing says you NEED two) PASSIVE speakers, and a powered mixer or power amp. The difference: speakers are the same, but the passive ones are just speakers, there's nothing amplifying the sound, that has to be done elsewhere. Powered speakers have amps built into them (much like keyboard combo amps have amps built into them). Something like this: http://www.musiciansfriend.com/navigation?q=yamaha+stagepas&st=
or a popular one, is two of http://bass-guitars.musiciansfriend.com/product/Accugroove-Tri112L-Bass-Cabinet?sku=480884
powered by http://pro-audio.musiciansfriend.co...-Professional-Power-Amplifier?sku=481541
Studio use. Powered studio monitors. I don't have links to any as I don't know anything about them, really. When I was researching keyboard amplification my application is live sound, so my knowledge (albeit second-hand) and limited research is restricted to that. But this is what you want, powered monitors. A good music store has keyboards plugged into a mixer which is then plugged into monitors, so if you want to hear what they sound like, check one out.
Difference between studio monitors and PA speakers? The distance the sound travels. Studio monitors are designed for up close listening. They are quieter (less hum), less durable (not designed for gigging) and can't be heard well from far away. Powered (or passive) PA speakers are not as suitable for recording, but can project into a room well.
Alright... almost done, I promise. So keyboard amps... they tend to be a jack-of-all-trade but master of none. They try to sound good, there's multiple inputs and sometimes multiple channels, so a mini-mixer, and small EQ. The idea is it is self-contained. Amp head, speaker, mixer, all in one unit. The downside? Kind of mediocre in all categories, and tend to be heavier than powered speakers. They are cheaper, and there's a reason. You get what you pay for. Pros and cons of a PA system (not gonna get into the pros and cons between powered and passive, don't know enough and the post is already too long): Pros: Sound good (I mean, REALLY good. I auditioned some low-end tapco thump speakers and they sounded amazing, so I don't know what JBL or Mackies or EVs would sound like, but I imagine better). Cons: You'd need to buy a small mixer ($100 or so) so it's not as self-contained. Also costs more. Again, you get what you pay for.
There is also an appeal for the Traynor K4 and moreso the Motion Sound amps. They are stereo imaging in a box... meaning two woofers, and two horns (small speakers for high frequencies). Not true stereo, since it's in one box, but up close it will sound better than a mono amp. They cost more than your average Roland amp. There's a reason... they sound better. Not as good as two PA speakers, but if you want stereo in one box, might be worth considering.