Plinius repairs

Sorry if this is a dumb question but have you tried reaching out to Plinius directly?

https://www.pliniusaudio.com/en/Customer_Support

It's a legit question. I did reach out to them and just realized that the guy I contacted with he's an authorized person in USA. 

Have you checked the set screw on the knob? Sounds like knob is pushed in to far and knob has friction with faceplate. If you loosen set screw pull knob back very slightly then tighten down set screw knob should move freely. I’m not familiar with your amp but have experienced this with some amps of my own.