Fatparrot, the tubes I picked up last week were just about all grey glass RCA 12SX7 tubes. I think that two of them were General Electric as well.
By the way, my favorites so far are a pair of RVC (RCA Canada) grey glass I picked up in the fall. They have the RCA 6SN7 grey glass kind of sound, which is a really fat midbass, lending some welcome (for me) warmth to the sound, in addition to a whole lot of slam. I use them in position V1, which is where I like them best.
It is kind of hard for me to describe their superiority over 6SN7, as the amps that I have that use them came that way. I was never able to listen to them with the 6SN7 tubes. It is a unique pair, and probably one of the finest sets of M60s in existence. They're certainly superior to my former pair, which needed the power supply upgrade as a start.
I will say that the sound of this pair is extremely liquid, relaxed, and natural sounding. Yet, at the same time revealing of astonishing detail, with a good bit more powerf in the bass. This particular pair has a lot of advantages over my former amps - better passive parts (teflon film/foil caps, etc.), a big bump in the power supply, and no negative feedback. There are a few other things with this pair of amps, such as test jacks on each output tube to measure it, the B+ supply has been modified with voltage regulation(I have no idea...) and a "pi filter" has been added to the driver stage power supply. Basically, they were sent in with the mandate to do every possible upgrade known, and that was that.
From other people who are more familiar with the before and after of the 6SN7/12SX7 with their pair of M60s, they are considered a super low distortion tube. I have heard that they take a lot of the best traits of the RCA (big time bass) and the Sylvania (superb midrange and treble) and put them into one tube. Hopefully, someone who has done the surgery can chime in with their opinions here, as I am beginning to drive past my headlights in this post. It's probably also time for Ralph to put me in my place...