Nice, I imagine this is a pretty simple mod but if you have a diagram you could share that would be great.
On the other frequency end, have you seen this problem?
Between that and your problem and the well known PWM/frequency interaction it seems like the 3340 pulse wave is quite problematic. I think @lookmumnocomputer was talking a while back about looking into not using it and instead making a pulse wave using a comparator on the ramp or triangle output but I haven’t heard anything more about that.