“Reasonable Synth” is a minimal instrument that makes a piano-like sound. RS responds to note inputs on every channel. It is useful to troubleshoot MIDI problems such as verifying that your MIDI keyboard is connected and working.
RS has no user-adjustable controls.
Because of its simplicity, RS is a great way to learn about MIDI and plugin signal flow in Mixbus. Add a chorus or phaser effect after RS, and you’ll get a reasonably good electric piano sound.