I recently had a bad experience with this particular tube - it doesn't fit well into the usual 6L6 socket in my two amps that use this style of power tube. Compared to NOS GE which pop right in and even new Sovtek stuff (eg, GT, Fender 6L6GC), these are a very poor fit. I've got no beef with Mesa, their amps are wonderful... but these particular tubes are a problem. There seems to be wide fluctuations in their tolerances with the STR440, particularly applicable to the middle tang. L and M (Long and McQuade) refunded me after a short discussion.
I've heard nothing bad about any of the other 6L6's (or any tube) rebranded by Mesa, but beware of this one. I will stick with my JJ's or Winged C stuff from now on.