I can share some observations and can relate to a lot of what you said. Not sure this is really helpful to anyone, but it's what I observe over time...
Gain and clean - it won't do clean like a twin just like a twin won't do distortion like this. On the gain side, I think it just shines in spades at any volume which became really became apparent to me the other day messing with an old blackface Bassman which can get some fantastic distorted sounds when it's at 115db+. The day after that, I plugged in the caz where I had it set on that low volume vid, same cab, and I was kind of dumbfounded how quiet it was set, like easy to talk over the amp and easy to hear the guitar acoustically right in front of the amp. Pretty amazing engineering progress there.
Sensitivity and all the other controls - sensitivity has a lot of character in it, like set high it has a compression component that doesn't sound like a compressor, and it changes the tone too. I would say you get more attack with it set low, and tone rounds or maybe loses some highs and has more "squish" as you go higher. Point being it affects the tone and feel of the amp quite a bit. The eq settings - I find I set them all over the place and it always just sounds good and surprises me some times where I have them set, like "yesterday" this sounded good, but today these other completely different settings sound just as good, just different. Aggression in general I like low, but sometimes I find I set it pretty high. The tone it imparts maybe makes me set the eq very differently, but I guess the point there is it really works together with the tone stack and master in general. It's different from I do with other amps where you sort of figure out the mid needs to be at 4 or whatever, it might one 7 depending on where everything else is set. It's just really tweakable.
Volume comes on fast - I think it's very linear. If you are playing through something really efficient like a V30 it's probably just more apparent acoustically.
fx loop - I plugged in a decimator the other day and that worked fine. I'm not much of a loop guy though. If I want post preamp effects, I'll do it more a wet dy wet config with a mixer or mic the cab and send that out to powered monitors or whatever. That Larry Carlton thing. I appreciate having the loop though, and will mess with that at some point with a straight stereo rig and 2 guitar amps.
Buffer bay - I have one too. Can't tell the difference between that and just some old boss pedal in line for the most part. I did once run a 50' cable from the board to the maz in another room and it was really apparent and helpful there.
Heavy - I have a lot of heads that are a lot heavier and appreciate this one.
OX - I had one for a little bit and returned it. One thing that really through me off was the size and weight of it. It's like another head. And that huge wall wart which is the same as the power supplies for the UA interfaces, I hate that thing and it just needs the power supply in the unit and one iec cable. Another thing that through me off was I don't have any cab that I've mic'd with anything that sounds like say the greenback 4x12 model. Didn't mean I couldn't get a good sound of it, it's just the amp would have to be set 180 from where I know a like cab with the same mics sounded to me. And the other goodies like the plate or 1176, I already have those in the ua interfaces so they didn't really add anything for me, and I don't think they sounded as good either. To me, the ox is a tool and could absolutely the right tool for the right person. But it's around $1200 too. For me, I kind of do the same thing using a Neve ($250) or warm audio di ($200) into a ua interface using a cab sim or whatever other plugs I might need, and the load box is a 4x12 or 2x12 in the next room with it's face on the floor. It does the job for me, and I don't think it sounds worse in any way. I also didn't like the attenuator in the ox as I felt it really altered the tone. If I had a different situation like a city apartment, it would be ideal though, so I don't really mean to knock it. Just not for me.
Back to the caz though, it just doesn't seem like an amp where you kind of learn where to put the settings to get a good sound, you can get all kind of good sound with the amp set completely upside from you last "good setting". I like and appreciate it more now than when I first got it. Particularly after switching through all my old amps.