Could be bad tubes,could also be something in the amp causing okay tubes to go bad.Make sure the tech is checking it out thoroughly,not just blaming the tubes and dropping a new set in there.I know Mesa claims there is no need to bias if you use tubes in the "right" range,but have the tech check to see that the bias is okay when he puts the new tubes in.I dont like the Mesa method of tube replacement,but with their system the tubes should fall into a safe ,somewhat cold range to avoid what is happening in your case.It seems unlikely that you would get 2 sets go bad in such a short time if something else wasnt amiss.It is possible,but if you are paying a tech to change your tubes he should look into it a little further after 2 sets in that short a time.I cant speak on the reliability of Mesa or GT's tubes since I never use them or any Chinese tubes,but they do sell alot of amps and tubes so they do work for a lot of people.Both Mesa and GT claim to test their tubes thoroughly and cull tubes that dont meet their expectations so to get 2 bad sets in that short span of time would make me suspect something else could be wrong.