I would clean all eight contacts on the tube prongs and recepticle. Use sand paper, I have a Dremmel kit with different grinding tips, ussualy use the stone one, gently though. Than use any electrical contact spray. Wipe off the excess fluid to prevent a short.
Also, the tube has to be inserted under straight 45 degrees, not to lean on any side. After rhat, I'd check the 8 lines in the amp, coming of the base for loosenes. And their respective connectors on the other end. If the problem persists, it's beyound what you can do, short of diagnosing and replacing a condenser. Good luck, and don't give up the tube sound that easily, what cans do you use btw with the DV?