Was playinig with Virta tonight in Ableton Live 9 and noticed a high amount of zipper artifacts are introduced into the audio when you automate the oscillator levels. Try using the LFO MIDI from M4L as a test.
I tried the same automation with FM8 Osc levels by Native Instruments and didn't get the same artifacts.
Is there interpolation that could be added to fix this?
P.S. Really loving Virta!
I can definitely add this. Thanks for the feedback!