Wow, lots of quick responses. Thank you very much! A little more of my thought process...
Since I mainly bought the AF3 as a multi-effects processor, I wasn't initially even concerned with the amp models, until I started hearing the forum banter: "This new FW is finally good enough for me to sell my tube amps", "I dialed in a tone on my AF3 that sounds exactly like my _____ tube amp", etc. And since I spent a lot of money on gear last year (new Majesty, 3K, used Tri-Axis/2:90 and a 2nd Recto cab, 2k, and of course the Axe), the last thing I wanted to do was spend even more money on FRFR equip. But I thought, hey, if it's truly that good, maybe I can start selling off some of this heavy gear as a start. But I want to convince myself, first. The effects are effin amazing, but I'll admit, I'm more than a little intrigued by the modelling components as well.
But I also wanted to hear, realistically, what all the modelling hype was about, so I figured the best 'dip my toes in the water' choice was headphones. I knew they needed to be relatively flat, so I read many posts, and many people here commented they hear the same thing from their DT 880's as from their FRFR monitors (although tbh, many also said the opposite.) So I figured the worst that could happen is the cans possibly wouldn't capture the models exactly. But the reviews I read generally led me to believe that the slight changes in the upper areas of the frequency curve weren't enough to matter (albeit, a subjective viewpoint, but for $250, I figured, wth.)
Now, one tone I do know is John Petrucci. And unlike the guy who was unhappy with the tones he was getting through his cans, partly because he was using an Ibanez to play Eric Johnson (I also read the other threads I could find on this topic), I have the best starting point, the Majesty. Plus, I've previously dialed in Pull Me Under (crunch rhythms as well as clean tones) on my Dual Rec, using a Boss CE-5 chorus for the cleans, so well that my musician friends (who are also DT fanatics) can't believe their ears. And he didn't even use that amp on that song!
So the first preset I auditioned on these phones was 'Petrucci Rig,' Crunch Me Under. It wasn't even close, to my ears! Yes, I read responses that suggested using global EQ to compensate for the frequency curve of the headphones, but what I'm hearing doesn't sound like an EQ issue.
I even doubt it's truly a perception issue, in that a guitar, played through a speaker enclosure, set at some distance from your ears, just isn't going to sound the same as through headphones, because the music I listen to on my phone, using these tiny earbuds supplied with the phone, sound freakin' amazing!
It's been suggested in a few places here to compensate with certain reverb settings, so I'll try that. And if anyone could show me, or point me to, the exact EQ settings I'd need to use, to compensate for the frequency curve of the DT 880's, I'd also appreciate that. Edit: I see now that's what that link was to.
So again, thanks for all the responses (and I'm going to read the link posted above also)! This place is great, especially for a newb like myself. I guess my overall, "WTF?" reaction to listening to the AF3 thru these cans, was how..., "saturated"(?), overly 'gainy' maybe, the presets I listened to so far sounded.