Alot of recordings from the 70’s and 80’s have probably been re-mastered though. So it’s still a crapshoot imo. And the gear that might have been used by listeners back then may, or may not have any relevance. (It might have some relevance for LPs or tapes produced in that era or earlier, for example.)
Neutral, transparent, low noise, and low distortion is still the safest bet though imo, at least as a starting point. And if necessary, you can adjust from there.
“Forgiving” is a term I’ve heard used for the HD650. So perhaps that’s the kind of thing you might be after. There are some noteworthy headphone users who prefer less treble or more rolloff in the higher frequencies though.
I am not one of them however, and still view this as a form of coloration/tonal distortion.
For those who don’t know what I mean by “neutral”, “transparent”, or “colored”, this video might be a good watch. It’s really about speakers, but many of the same principles Toole discusses here (and in his books and articles) will also apply to headphones.
