If I understand you, subbing a 12au7 in V4 would limit the power output???
No. Compared to the 12AX7 in there now, a 12AU7 has a much lower voltage gain factor, so the gain value for that stage is reduced, but a much higher plate current/output power capability, which in turn will drive the output tubes harder, and with Mesa's insane amounts of preamp gain, it's not like the amp actually ever gets quieter. You'll notice the noise floor of the amp at idle will drop and it will have more punch and lose less low end at high volume, and because the drive section now has greater power, it will drive the power section to greater output power. The difference isn't earth shattering; it's more a change in the feel of the amp, but it will be noticeable.
Meantime it sounds really good just not as loud as I expected. I haven't owned it long. Replacing the original 30 year old tubes seemed a reasonable proposition.
The thing is, the modern accepted concept of "short" lives of tubes is little more than repeated marketing hype started way back when folks like Groove Tubes and Mesa figured out that they could convince folks to pay a premium for ordinary tubes with fancy labels and performance ratings derived by measuring their gain at a set bias current and plate voltage, which is generally much lower than the actual operating conditions, ergo the ratings are almost meaningless. Power tubes, even driven very hard, generally have the same life expectancy as power transistors: let that sink in for a minute. How many people with solid state amps talk about "retransistoring" their amps other than replacing defective ones? The first things to need replacing in most tube amps are the electrolytic capacitors and large wattage resistors, as they change value with heat and age. The fact that tubes have sockets makes them an all too tempting catch-all for any kind of amp ill, but the REAL truth is that if a power tube is bad or worn out in a tube amp, something else in the amp killed it, and until that problem is addressed, dropping in new tubes is just as likely to make things worse or at best case, ruin a new set of tubes.
It would be a three day affair to get a tech on it in between the driving. Courier service is more risk than warranted. If I knew of a truly worthy tube tech to take it to I could plan vacation around it. Mesa NZ told me to play it until it acts up.
Mesa's "service" network is based on sales, not product expertise, and their answer reflects that. Any competent tech who knows his way around any old tube Fender bigger than a Bassman should have no trouble measuring the output power, installing a bias adjustment, and setting the bias for maximum clean power. He can also tell you just how healthy your output tubes
actually are. I know a REALLY good tech in Australia; I can ask for a recommendation from him for someone in New Zealand.
...and back to my point about Mesa's exaggerated ratings, the fact that your roughly 100-120 watt tube bass amp has an equivalent volume to a s/s GK MB200 is normal. The things I am telling you about will enable you to truly hear what the amp is capable of doing and if you like the amp's tone now, you'll like it even more once you tweak it a bit.