I've got a pair of old weird speakers that I think my grandfather built in the 60s. Else he bought them at some junk sale from somebody else's weird grandpa... Anyway, I've really been wanting to find maybe a couple cheap little guitar amps to run them as a completely different perspective from my real speakers. A guitar amp (not talking about guitar speakers, which are basically what everybody else calls woofers) usually has a severe mid scoop when the tone knobs are set "flat". Sometimes turning the mix all the way up and the bass and treble down can get you close to flat, but it is kind of just a matter of changing a couple caps out, or even just bypassing the tone stack...
A keyboard amp would definitely work better. They're often designed to be clean and flat and reasonably full-range. It not going to be anywhere near ideal, and decent actual studio monitors will always be cheaper anyway.