I own eight (8) bluetube pre-amps (original first release) and need to replace the tubes in some of them. I recently bought some 12ax7's and put in a few of them only to find that the amps blew the tubes instantly, so I put the original tubes back in them. They still work for the moment but are testing out on the weaker side of things. What tubes are suggested to be used in them before this becomes an expensive experiment?