That’s correct and is a “feature” of how filtering works on the SID. There is a post about that somewhere on the forums (though it may be rather old by now). If you want to have a nice filter on top of polyphonic sounds, you may want to look at the SSM2044 stuff. I assume that behaves better with the addition of multiple notes. Disclaimer: That is raw assumption - I have not finished (or really even started) my own SSM filterbox so I don’t have any proof 