Carvin's placement of the 12AX7

You mean you actually got a response from Carvin? They closed down their amp business back in 2017.

"For added warmth, take advantage of the internal 12AX7 TUBE that utilizes two stages that precede the power amp section."
I think you're misreading their statement, which is also marketing-centric. A 12AX7 is a dual triode device and most preamp designs make use of both triodes. So it's not two stages before the power section, but rather a circuit that uses both triodes before the power section. Note that many B1000 were sold and shipped without the 12AX7 installed, and the amp includes a bypass switch on the back to toggle between tube and solid state preamps.
Amp manufacturers like to tout having a tube preamp to add "warmth" to the signal. My understanding of "sag" is that it is mainly a function of the output section of an amp, not the preamp.
b1000-user-manual.pdf (shopify.com)

 

Yes, as the manual and blurb says, it's a 12AX7 preamp, with a switch to choose between tube and solid state preamps. There are no other tubes in any other part of the amp.

Yes, the majority of manufacturers will offer these hybrid preamp models and tout "tube warmth" as the benefit. How well each manufacturer realizes this is a matter of taste. My old Tubeworks RT-3300 was one of these hybrid designs and there was absolutely a noticeable difference in character and tone between tube and solid state preamps.

 

It depends entirely on the intent of the designer, and their skill in translating that intent into hardware.

 

Thinking way back, I seem to remember the B1000's tube worked kinda like Hartke's in their X500 series.

Basically, it's not just the tube that's giving the amp a voice, but the entire circuit the tube is part of. I remember that as long as you put a modern tube in there (it's not designed to use vintage tubes) that it pretty much always sounded the same no matter what you put in unless the tube was actually defective. I'm not even sure if the Drive control was actually using the tube to "drive"... it didn't seem like it. You could bypass the tube and still get it to drive.

It didn't play well with my upright bass (I can see why in the specs, if they're accurate) but it was fine for my bass guitars.

I remember it was a pretty warm sounding amp. Not clinical at all and that the Sub-Bass control was kinda questionable in usefulness, but overall a good sounding amp. As long as it works properly it should be fine. I'd be a bit nervous about replacement parts for it if it ever needed repair, but I'm probably overthinking that.

 

I played one of those amps at their showroom here in San Diego. At the time, it was the best sound amp I had played, very nice sounding. I agree with the JJ Tubes, I used them in my Hartke HA3500 and Audio gear. I'm going to put one in my Carvin BX1500. Nice sweet sounding tube. Enjoy!

 

In designs that allow for it, you can turn the preamp up until you get a little sag if you want it. It's not a dramatically awesome as turning up a tube amp till the power section starts to sag, but it can be done much quieter