Yeah, the STR440 are quite different compared to the other two. Depends on the amp in question when I like them or hate them.
I was just making a generalized comment based on what I personally experienced in the past. I have not ventured into the IR stuff, but power attenuators, tried that. Did the 4CM thing too, some multi-effect units only to find it had issues with latency and not working out so well when you added too much into the processing. No more for me. I prefer to keep it simple and would rather use separate FX units in the loop as they only do one thing well and that is process their intended function. For the other units, they would have to be a multi-core processor or several that can run at the same time without creating digital artifacts or worse. Some of that stuff has come a long way so perhaps one day I may venture into that world, that would be the day tubes are no more.
As it seems, we end up being the experiment by the vendors who sell the product. The old descriptions before Gibson took over, did identify the Mesa amps that were selected for what tube, Mark V90 STR441, JP2C STR443 and MWDR STR445. When I got my hands on the Mark VII, I was surprised to see the STR445 tubes in that amp. Tried them in the JP2C and thought they sounded just about as good as the STR448 and the STR415. My JP is a 2016 so it came equipped with the STR440 tubes as did the MWDR I bought in 2018. Both amps had grey coded tubes. Tried the STR445 (greens) in the MWDR and did not like them at all. I do have some in yellows to try in the MWDR. For some reason or another I favor the STR440, I sort of like that pissed off bee's nest growl you get with the MWDR, it was not so ideal with the JP2C but it did not take away from its performance either.
I can agree with those who claim it makes no difference but yet disagree at the same time and say it does. Depends on the amp and may also include how it is used. I did try the power soak thing, Rock Crusher Rivera unit. Not bad with the Roadster. Made the other amps sound sort of ***** or boxy.
I moved away from active pickups as they were not providing what I wanted as I kept getting the same characteristic from the guitar when I rolled off the volume. Passives will change when you alter the position of the volume pot more so than with actives. Just my 2cents.
I have nothing against actives, use them as you see fit. I still have one axe that has a set of EMG's but they are the hybrid types with exposed pole pieces, 57 /66 set in a cheap Squire contemporary strat. I like those better than I thought I would. The neck is half decent for a $400 instrument. Body is made of Poplar wood and obviously made in China. The original electronics were junk copies of the EMGs from the past. All that had to go. New pickguard as well from Warmoth as that was one place I could find pickguards made for a FR bridge.
I have had a few guitars in the past that had the EMG 81 back in the late 1980s before I bought my first boogie. I believe I took this picture in 1985 while I was still in high school, back in the day it was cheaper to develop black and white film. Bummer, I had to sell the Spotlight Special to finance my divorce from the first wife in 1999. It played quite well for such a heavy guitar. The Robin on the left had the EMG 81 or some sort of EMG pickup. Sold that guitar in 2007 with most of my other gear as I gave up on music (I thought I did but got back into it in 2012).
A few of the Carvin guitars have active tone controls, standard passive pickups. DC200 (replaced the guts with the DC400 electronics). Three DC400 guitars. That is what I had starting with the Mark V90. The old Charvel Model 4 I had (removed the active tone controls from that one and installed just a SD pickup in it, may have looked like hell but played well).
Though of building my own Super Strat, well that was interesting. Started out with EMG David Gilmore set, wanted something different so I changed the pickups, and again wanted something different.
Thought the Fluence Strat set would work out, hell no. OK if you like to keep your amp on the clean channel but did not favor distortion much. The exception was the JP2C as I can run just about anything into it and it will sound good. That did not hold true with the TC100 or TC50 or any other amp. the Fishman Fluence strat set was a waste of money. Sure, I was able to get a strat sound but like I said, only on the clean channel. the EMG SA or the RA were much better for higher gain applications. I eventually ditched the actives in the project guitar for something passive but yet different that your typical single coil. Zexcoil legacy juicy buckers.
So how does this all relate to preamp and power tubes? It has an impact on how the amp will sound, how it couples to the guitar through the speaker cabinet. Once that coupling is taken out, you lose that feel. It may still sound good though. Just getting used to wearing headphones is the hard part. I had more enjoyment just using the attenuator so I could crank up the Roadster into its sweet spot, otherwise it would be too loud to do much with it.
What transformed that amp into something different was a shotgun approach to stuffing the entire preamp with some old Mesa tubes from the 1990's. They were leftovers from my band days with the Mark III. Also grabbed some Ruby branded versions from Doug's Tubes when he had them available. Never expected that kind of transformation. Sometimes things will pan out and sometimes they do not. Now I do not need the EV speakers and can make due with the V30s. Not a total loss, that cab is amazing with the JP2C or the Mark VII. That was my first 412 cab, did not work out with the Mark IVB, total flop with the Mark V90 (it had issues), so I ripped out the V30 and replaced with EVM12L black labels. That only made the ice pick far worse that it was.
The 12AX7 comparison charts almost resemble the pickup selector guides you may see on some vendor websites. Take it with a grain of salt as the tube chart does not clearly indicate what type of circuit it was used in. How many tubes of the same type were tested as I have found not all of the same type sound the same, including the Mesa branded JJ ECC83s tubes.
https://www.amplifiedparts.com/tech-articles/12ax7-comparison-current-made-tubes
Then comes the power tubes, would be nice to have some sort of comparative chart for those. First time I ever saw this one. Based on the description, they were tested on a single ended amp, I assume true Class A mode then. Not seeing much here. What about early distortion characteristics? That is one important factor to consider when it comes to power tubes, is it Class AB or Simul-Class. That also indicates if you need more headroom or more tube distortion.
https://www.amplifiedparts.com/tech-articles/6l6gc-comparison-current-made-tubes
Does it make a difference, not always. When the tubes you are familiar with end up in the out of production category (NOS), it would be nice to find what is comparable or as good in current production tubes. I was going to say obsolete, in general terms, tubes are an obsolete technology but necessary for guitar amps that sound good. Eventually that too will fade away but may as well enjoy them while we can.