You can very much tell, believe me. At stage volumes modeling amps get... problematic. Muddy, unfocused. I worked at a rehearsal/recording studio in LA that did a lot of industry showcases, and the head tech hated it when anyone tried to use a modeling amp in a showcase because it took massive effort to make it fit in the mix.
What "Modeling amp"? Not all "modeling amps" are the same. And there's no inherent reason why a modeling or SS amp has to be "muddy or unfocused".
This kind of nonsense is based on a fundamental misunderstanding of signal theory and digital technology which is why it annoys me, especially when people who are theoretically very intelligent subscribe to it.
"Experiences" doesn't change how the theory works. The traditionalistic nature of guitarists really gets me some times.
The effect that causes digital amps to be more difficult to hear in a live situation is the Fletcher Munson phenomenon. Tubes tend to compensate better for this effect by changing their character while cranked. However, this just means you need to re-tweak the settings on the amp, generally, and not all amps will be effect in the same way. Also the AD30VT has a tube in the power section anyway to make it respond in more or less the same way, and newer Line 6 stuff has compensation built in. It's just an EQ trick. Plus using modeling you'd often go direct out - so the effect doesn't always apply in this situation. You're talking about a "digital" sound somehow not being as easy to mix which is silly.
If you're mixing a digital amp you just need to boost the upper mids. Or just use an EQ pedal.
A 12AX7 is a preamp tube.
But it's USING it as a power amp tube. Meaning it involves a tube but is using it in a way that evidentally hasn't been used before, so you shouldn't judge it by the standards of previous modeling amps.
The reason it does this as far as I can tell is that you can saturate 12AX7s at very low voltage, but it's being used in the power stage of the amp. It's got a special circuit that emulates class A, B, and Class AB amps. You can actually get a proper "cranked tube" sound at low volumes. As far as I know using an actual power tube would require an awful lot of power. I'm not sure exactly how it works, it explains it in the Valvetronix manual. Even without the tube(direct out) it still sounds great.
I would suggest you target your amp when you're looking at this problem - get a decent tube amp (there are cheap ones available!) or find a good non-digital solid state. Things will start looking better.
There's nothing wrong with my amp. I love my amp. My problem is that the kind of sounds I'm after are generally only found in amps that aren't quite so cheap, or don't exist. The only amp I was really interested in that I have money and space for was the Tiny Terror but I use too much delay/verb for it to be practical. There's nothing wrong with modelling amps inherently. The AD30VT is miles beyond the likes of the Spider. And it handles high volume pretty well too, if anything it sounds better like that.
If a Solid State amp can sound "unmuddy" then there's no reason a digital amp can't either. Digital amps aren't quite as good as real tube amps, but they're better than the vast majority of SS amps too.