Anyway, I think it says on your site that Sylenth1 is already free of aliasing
Either way, it is already way better in this respect than most other synths I have tried to far. Maybe because I don't use extreme modulation or very high notes.
So, assuming cpu load might double, it should be enough to use the HQ setting only during mixdown, right?
Is a real last-note retrigger mono mode on your to do list? There is some space left for a third option on that switch in the corner

