Specifically regarding the MS-20… a popular patch configuration is to take the signal out of the main or headphone outs and pass it into the filter, or the external instrument processor (and then to the filter). This creates feedback and can fatten up a sound nicely. You’d then use the other of main/headphones to take the signal to your speakers, or mixer etc. The two outs are controlled by the same volume knob.

I mention this because your initial plan is to connect the synth directly to the speakers. This means you’re likely gonna use the synth’s main out knob to control overall volume from the speakers. But doing so, you’ll also be attenuating the feedback signal, which will monkey with the sound you’re making, if you’re doing the feedback patches.

A mixer will help get around this. It’s also not a big deal as no-one’s gonna force you to make feedback patches and there’s still heaps you can do with an MS-20.