Ok, did some comparison with your very own video.
First off the settings I copied on my box:
http://www.falckenberg.com/downloads/Quaverato/Quaverato-Settings.png
Secondly, the waveform as recorded, both from your video and my Quaverato:
http://www.falckenberg.com/downloads/Quaverato/Quaverato-Waveform.png
Here’s the audio off your video:
http://www.falckenberg.com/downloads/Quaverato/Quaverato-Video.mp3
And finally from my box:
http://www.falckenberg.com/downloads/Quaverato/My-Quaverato.mp3
On my recording there’s always an underlying constant tone, it never shuts off completely. And that’s with wave shape set to square wave, full depth, spacing in the middle, harmonic mix where the effect is the strongest (my pot ends up in the 11 o’clock position when the separation is strongest).
Should I replace the transistors in question?