Oyster,
I don't think it makes much sense for Mesa to sell crappy tubes. I've had a limited sample so I couldn't say that they do sell crappy tubes - but 5 of the last 6 EL34's were not useable. Maybe a bad day at quality control? I don't know. It is worth keeping in mind that Mesa wwaaayyy over-charges for the tubes that they make you buy.
After playing phone tag, customer service is sending replacement tubes, and seems very professional and supportive. I requested new 6L6's instead of the EL34's since I've gotten so many bad ones.
I'll report when the new tubes get here.
By the way Oyster - are you using those EL34's in a head or a combo. Combo's tend to make the tubes rattle.