If you inserted the JJ 6SN7 with the keyway properly oriented, this should not have happened! Typically, JJ uses thinner-than-standard contact pins on their tubes. This results in a looser than normal fit. My guess is that you may have forced the tube in incorrectly, jamming the keyway in, but out of its groove.
Very interesting. Over the years, I've used 3-5 different brands of each type of tube in these amps. Other than these new JJ 6SN7s, my new JJ 5Y3s are easily the tightest fitting tubes I've used ... and the 6SN7s are MUCH tighter. Meanwhile, the JJ 2A3-40 is the loosest fitting 2A3 I've used.
In any case, I managed to get that stubborn tube out. I wrapped it in several layers of cotton fabric for protection and pried gently between the tube base and socket with a thin but stiff plastic spatula on each side. I'll be away from these amps for the next week but will try to follow up with more info soon.
Thanks for the help!