Please don't get a guitar or bass amp for your keyboard!!! But don't just take my word for it, go try one and you'll be convinced as well!
Guitar amps are made for guitars. Guitars have a much narrower range than a keyboard and are very mid-rangy. There is typically only one or two speakers -- usually 12", or possibly 10". You'll get a very "honky" or "punchy" sound with no high end. Also, guitar amps are designed so that when they're turned up to a certain level, they distort. This is considered a good thing on a guitar. But imagine playing a piano sound and having it distort.
Bass amps are even worse. They're designed to play very low frequencies and that's about it. Although there are a handful of high-end bass amps that actually have a horn/tweeter, you're going to get a very muddy sound.
Keyboard amps can sound very good. They're made for keyboards and can reproduce a full range of frequencies. Some can get very loud as well if that's a concern. I'd encourage you to try before you buy.
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