This depends upon how you use the amp and I should have clarified this. When I used the term "V1" I was meaning more in the generic sense of the word as in "the first preamp tube" = meaning the first tube your guitar signal hits. Fender amps of this era DO NOT use a labeling of tubes such as "V1" or "V2" ---there simply is no such nomenclature on the schematics. So, in the absence of such labeling, we amp dudes often refer to "the first preamp tube" as "V1" even if there really is no such thing in this case. But we seem to know what is being talked about. (Now you can send in and get your magic decoder ring!) Most other manufacturers DO apply such a label to the tubes...which is completely arbitrary. One mfr could label the GZ34 as V1, another might label the first little tube V1 and it would not matter.
If you want to lower your gain which will have the effect of A: giving you more rotation before breakup on the volume control (meaning, you will be able to turn that control up to say 8 instead of 5 before the amp starts to break up--but this does not affect watts at the output) and B: kind of smoothing out the response of the amp, then swapping a 12AY7 or 5751 for the stock 12AX7 in "the first preamp tube the signal hits" is a very painless and obviously reversible way to achieve this.
On a 6Gx or AAxxx (meaning white or brown or black separate-head-style, not Tweed) Bassman, the "first" channel is labeled "BASS INSTRUMENT" and if you are plugged in there, the first tube your signal hits is the rightmost (as viewed from the rear) little tube, the tube most distant from the power tubes.
Should you use the amp for guitar and you plug into the "NORMAL" channel then "the first preamp tube your guitar signal hits" would be >>"V3"<< the THIRD physical tube, starting your counting at the right.
So if you NEVER play through the BASS channel, there is no need for or benefit to changing the first tube....you'll never hear it. If you only play through the NORMAL channel and wish to do this change, swap out >>V3<<. The third-from-the-right tube. For the normal channel, that is "the first preamp tube".
You should probably NOT change out V2 = post tone stack control gain recovery nor V4 (phase inverter) But you can, if you want to. The 12AU7, 12AY7, 12AT7, 12AZ7, and 12AX7 tubes all have the same basing diagram and current consumption and voltage capability and swapping them out in any fashion you can imagine will definitely not hurt anything. If you believe your amp sounds better with a 12AU7 phase inverter, by all means, swap it out. The 12AU7 is about the lowest-gain member of this family....who knows, maybe if this "overload" problem you mention on big fat notes is an issue, there's a case to be made for making that change on V4. You can't hurt anything by doing so. You either like or don't like the result, leave it or swap it back.
Many people like using a 12AU7 as the reverb driver in Fender reverb amps. More (actually less) power to 'em. On Fender reverb amps, I have found you really can't turn the reverb knob past about 3-1/2 anyway without going full mud. Answer: Swap in a lower gain 12AU7 tube for the 12AT7. Easy, obvious solution. Also on Fender reverb amps, most people never, ever use the normal channel. Ever. Such people who wished to try the 12AY7 trick on the reverb channel would swap out THE SECOND little tube.
If you consider that any of those four tubes could be changed to 4 or 5 different types (12AZ7 is rather uncommon) and of those types, 6-10 different brands (if you think Sylvanias sound different than RCAs and different from JJ's and different from Telefunkens) there are hundreds and hundreds of different possibilities. There are lots of adherents to this type of thing and lots of web pages devoted to this particular religion.
