I'd try replacing it. Come in from the back, just take out the power tubes, remove V6 (see the manual--it's the small tube second closest to the fan on the left when you're facing the amp from behind), and swap in a spare 12ax7. Then put the power tubes back in their original spots.
You're asking how to verify pre-amp tubes. Tapping on them can sometimes reveal a problem. But it's an unreliable method. The best method is just to replace them. If a problem goes away after replacing a tube, then the tube was the problem. The second best method is to test the tube in a tube tester, but most of us don't have one of those lying around. Tapping a tube probably won't give you a false positive, but it might easily give you a false negative.
If you don't have a spare pre-amp tube, try this: Remove V2 (see the manual), and put it in V6. Now try to recreate the problem. Do not turn on channel 2. The V2 pre-amp tube is only used on channel 2, and so I suspect that the amp will work fine in channel 1 or 3 even if there's nothing plugged into V2. If the problem goes away, you found your culprit.
Alternatively, to play it safe, just put your current V6 tube in V2. If you find that you can now use the loop on channel 1 without problems, but now channel 2 squeals, you've found your culprit.
