You need to know that I consider Paul’s finding absolutely ridicules. The 6C33C are very reliable tubes with surprisingly low for Russian tubes defect rate. You juts was unlucky or most likely the person who sold to you your party of tubes sold you bad or used up tubes. The tubes from 196os were slightly better sonically but inferior mechanically. Stay with tube from 80s and you will be very rare seeing any problems.
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche