For playing music with other musicians an amp is good, like a keyboard amp (Roland, Fender). A bass amp (Ampeg, Eden, Fender, etc.) will do in a pinch (especially if it has the “ice pick” horn). Guitar amps don’t have a lot of low end or sparkly high end, but maybe that’s fine if you are going for a funky, midrangey sound. My favorite is just a powered PA speaker or two (Mackie or JBL, QSC if you have extra money), and a little mixer (like Behringer, Mackie, Soundraft) to mix a couple things (synthesizer or two, drum machine, computer/iPad, reverb effect).
Studio monitors (KRK, Yamaha, Event, etc.) are what you want for recording and mixing music, the idea that what you hear coming out of them will be similar to what people will hear out of their stereos. I play synths out of studio monitors (via a mixer that is also an audio interface), because it is all set up for recording, but, for just playing a synth like the matrixbrute, an amp with bigger speakers will sound impressive/full. Extra points for stereo, like a stereo amp (I like the Roland JC120), or a small stereo PA, like 2 powered speakers or floor monitors and a little mixer.