Thanks for your thoughts and suggestions. I think I've got all that covered, but I'll go over it and add some more detail just in case anybody wants to join in the fun at home.
Re: MIDI – I'm just playing the notes by hand so far, but I'll try using an external trigger and see if that makes a difference.
Preset mode is off. Velocity and aftertouch are off, wheel-mod destinations all off, source mix set to 0, LFO off and zeroed, same with poly-mod, OSC A & B, glide rate, mixer settings, vintage set to 4.
Filter settings: rev 3, cutoff 0, resonance 6, envelope amount 3, keyboard off, attack 0, decay 5, sustain 0, release 5
Amp: attack 0, decay 5, sustain 0, release 5
The variation is more pronounced with unison on, but still present with it off. Also, if I don't set unison high, the overall volume is so low that I have to crank the master volume to get a usable sound.