My definition pretty much along you guys line
BUT "clean" and "headroom" are not as synonymous as it seems (well my definition).
To me Vintage Clean is warm, and has a slight break up but not so noticeable especially if you are using alnico speakers.
Now headroom I picture => crystal clear, "Mark I or II" Class A/B type (not Simul Class) with EVM-12L speakers. This is great for funk or maybe even reggae,
I dunno know. I know this is subjective and I'm speaking behalf of my opinion of "clean" and "headroom" ...
I remember in an article a pickup winder, maybe Seymour Duncan, customer asked him to wind him some "clean" pickups. So Seymour Duncan thought: "Clean => medium impedance, plenty of fidelty" so he rewound him so low gain pickups.
Then customer disappointed in the pickups said: "I mean 'Santana' clean, you know how he holds notes to sustain, and he doesn't clutter his notes, that's the 'clean' I want."
Seymour Duncan replied back: "That's not necessarily my meaning of 'clean' because he actually plays very distorted with massive overdrive."
So misnomer of the word clean[?] :?