..is there a good argument for solid state into my 212 cab?
The main one is that you get to use any and all AX8 amp models that sound good with that cabinet.
How it breaks down IME:
AX8 Into quality SS Amp (linear response, high slew rate) into Cabinet = optimal results only with those amps & effects combinations that work well into that particular cabinet. IE: running Fender Amp models into a cabinet with V30s or Greenbacks works fine but don’t sound as authentically Fender as a Jensen IR and FRFR - if you need that. That said: This makes for a great sounding rig that is a more reliable and convenient variation on how many of us have played for decades. Plus it has the added benefit of being more portable and delivering a cranked tone at any volume.
AX8 into tube amp into cabinet: IME, a tube power amp is the least optimal - if you need authentic modeling. It only works within a narrow range of choices. Might as well use a whole amp and the AX8 just for effects (which I do with my remaining tube amps). Few or none of the combo AX8 amp models + tube power amp will sound and respond just like the modeled amp or hardware. Power amp modeling needs to be turned off and a fair few advanced parameter adjustments need to be dialed in to get the response to match up well: and for each amp model. No benefit over a SS amp for anything I do: but it is possible, and I can see where it might be quite useful for some tone meisters with a tube power amp and cab that really does it for them and aren't married to amp model authenticity.
AX8 into FRFR = All Amps, Cabs & Effects closely replicate their real world hardware incarnations. Lots of IR choices to get it "just right" for any amp preference.
Highlighted for emphasis...thats a pretty important factor in everything you typed up there. I don't find myself limited to any subset of the amp models based on my PA/cab. I find the modeling side of it interesting, but not important to me. In other words, if I pull up a Bogner model and I enjoy playing with it, then it piques my interest in finding a real Bogner to play through at some point. If I do find that real Bogner and its nothing like what I was getting through the AX8 rig...I really couldn't care less. I'm only interested in getting a tone that works for me, doesn't matter one slight little bit if it winds up being 'authentic' to whatever my chosen model is or not. I only went with the Engl poweramp because I used to have an Engl head and I loved it...saw a good deal on the PA & jumped on it. With hindsight I wish I had put effort into finding a good SS poweramp to try out first, but all things considered I don't feel like I'm missing out on much for my use.
So...I'd tend to agree if you're after model authenticity and having more/all available options in the modeling (i.e. poweramp modeling) then you might need to look into SS. If you're not fussed about any of that, then you should just find the poweramp that works for you...if it has tubes then you go down that road, if its SS then you go down that road.
My .02 anyways.
I totally identify with your post. As it is, I've only bonded with a small percentage of amp models in the AX8 and don't expect to bond with more than half (if I'm lucky) regardless of what recording station or FRFR I have set up.
This doesn't mean that I don't want to get the most out of my AX8, but it does mean that if I'm happy with the sound, I kind of don't care what's making it. As it is, I've set up a bank of presets with the sag turned off and the cabs removed - running into the power amp on my 20 watt tube head and through my 2x12 cab. All eight patches sound some version of fantastic, and absolutely sound real. Until I started this thread I had only thought about getting tones that I liked, not really worrying about how authentic they were to the amps they were modeling. - A valid point, just not necessarily my priority.
I do see a reason to sim the power amp with my AX8 and just amplify that with ss into the 2x12. Yet, when I was setting up my eight patches, they all sounded like a real amp playing through my 2x12. This is my ultimate test for the AX8 in this scenario: does it sound like a real amp in the room with me or just a mic'd studio amp. It sounded real - impressively so.
I'll keep thinking about this. I honestly can see a reason to go either direction.