I have the 88, and have not had the problem, yet. Checking the bias and tubes sounds like next steps, and should have been done with the tech who you used.
Before you spend and more time and money, best to call on Steve, at cayinusa.com. Steve is the distributor in the US for Cayin and is also a very talented tech. I've seen his work bench and it has all the tools that a great tech should have.
Tpreaves makes a great point about finding techs who know tube gear. Best to go to the manufactuerer/disbributor from the start. In the long run it is less expensive and gets the problem solved the first time.