It's complicated...
Each 12AX7 has two itty-bitty amplifiers inside it, and they are not assigned the same function. For example, V2(B) (the second side of the second 12AX7, is assigned to the 1st gain stage Clean and the second stage Overdrive. You'd have to ask Mesa exactly what they mean by "Clean" and "Overdrive" in a two-channel, four-mode amp.
The very first 12AX7 is the tube closest to the input jack. This is V1.
The second half, V1(B), is the Channel 2, third gain stage.
Sounds like as good a place as any to start.
In any case, you are going to want to troubleshoot by replacing tubes with known good ones, ONE-TUBE-AT-A-TIME.
If you have money to burn, just go buy five 12AX7's of choice and replace all of 'em.
You won't know which tube is the bad one, because you didn't troubleshoot ONE-TUBE-AT-A-TIME, so, unfortunately, you will have to discard all the old tubes.?