The H90 doesn’t recognize MIDI Note input, so a ‘dumb’ MIDI controller keyboard won’t work.  HotSawz generates its own sawtooth oscillator(s) from mono audio input.

Something like the Bass Station does work, using its audio out for processing, and MIDI Out for CC control over HotSawz parameters.

It works very well with simpler waveforms as an input, for ‘stacking’ synthesizer voices.