Primeholy,
Congrats on the .22, like everyone is saying, it's a great amp.
About the SPAX7, as Lesgoldy said, it is indeed Mesa's part number for a high quality 12AX7. He is also correct in saying they reccommend it in the V1 position in most amps, and it is no difference in this case. If you look at the schematic for the Studio .22, V2 is actually the first valve the signal from your guitar sees and V1 gets cascaded infront of this when you kick onto the lead channel. That's probably why the SPAX7 is in V2, not V1 like in most other amps.
I've also got the lead drive/lead vol mods done to my .22 and it makes for a MUCH more versatile amp, especially if you play mainly lower gain stuff. My extra knobs were put on a new panel around the back, hung of the vent infront of the reverb tank; keeping the stock look round the front. My serial's pretty close to yours too, SS3618.
I also want to ask, do you experience any problems with the reverb unit feeding back now yours is modified with a larger tank, and better positioning? Mine still has the stock reverb tank and feeds back when the reverb level is above 3.
Regards, Chris P.