Welcome to the board. Hope that was a typo and you ordered a 12AT7. It already has a 12AX7 in it, so that would only be a spare.
If you did order an AT7, I strongly urge you to play the amp a LOT before doing the swap. Get to know it inside out. There's an amazing amount of tonal and gain...
Couple more days of experimenting from this base. There are some great tones of the D style just a tweak or two from the base settings listed. Some really cool stuff! Switch to Mark IV and add a little gain... add just a bit of treble and/or presence... WOW! Back to Xtreme, and more...
FYI, those settings are VERY nice! But (maybe because it's a Tele forum) I needed a bit more treble or presence. The pic posted was low volume. Turning the master up really opens the amp up and fills out the lows so some more treble or presence was needed when I tried it with my PRS NF3...
You don't need to solder or do anything tricky. You could just clip one leg of the cap. But if for some reason you don't like it, you'd then have to replace it since it would probably be ruined (the cap you clipped the leg on). But it's super easy. Or should be.
Yeah, don't be fooled by the 10 watt setting. I tried some 10 watt, master dimmed settings... louder and more ballsy than many 25 watt amps! Big glass/big iron!
That's really interesting because I haven't tried it yet since the AT7, but I found some great tones in the past with the W curve on C3. Your going the opposite way. I've never even tried that.
That might explain why I didn't like the new Mesa AT7 in the big Mark V nearly as well as the old Mullard AT7 in the V25! Nowhere near the change in tone that I heard when I dropped the Mullard in the V25. Guess I got to track down some more NOS AT7s. 8)
It's not quite that simple, but if the amp is working for you as it is, that's great. I changed it in both of mine and may never go back. And I use the high gain channel the most. They just sound better with the AT7 in V4 of the Mark V or V2 of the Mark V25. Turn the gain up just a little...