I am a bit confused abut my MkIII long head that I bought a couple of weeks ago. It has a "Simulclass Power" sticker above the power cord on the back of the chassis, but has the "fullpower-1/2 power" switch on the front. It is has 4 6L6's in the power section (STR-430's in the outer sockets & STR-454's in the inner sockets) I called Mesa and asked about it & they said more than likely it is simulclass. But I read an earlier post concerning the same topic & they said that the output transformer #'s have to be either "SC152019B" or "562300" for it to be a simulcalss amp. My output transformer number has "105-162318" with "EIA 606 434" stamped below it. So, needless to say, I am confused! If it is a simulclass I'd like to know for sure, for I'm just dying to put a pair of EL-34's in the outer sockets and hear how it sounds! Help! I'd also like to know what the difference between the STR-430's & the STR-454 tubes as well. Thanks!