Timbre is a reflection of frequency response, though. Which can be changed.

And no, I'm not saying that frequency response is literally everything, but timbre is a property of instruments, and its essentially the properties of an instrument that make it sound like... well, itself.

So for good timbre to be correct, that means that the tonality of the headphone has to be accurate to your ears in that respect, which is, again, frequency response.

That's why I don't put much importance on a headphone's timbre, because if I have issues with that specific property of the headphone, I can just fix it with EQ.

With the 6XX, it has recessed treble (EQ thankfully helps), really muddy bass with no texture or definition (EQ hasn't helped this, have tried for a while), and all the frequencies smear together compared to other headphones because the driver is over-dampened. Yeah, the mids and timbre are good, but the rest of the sound suffers as a result. I'd rather just get a headphone without these issues, and tune the mids to be similar to the 650/6XX, so I can get its benefits and the rest of them as well.

That's why I disagree so heavily with the recommendation. It's way too expensive for what you get, there's better-sounding (I know, subjective, but at least in terms of the rest of the frequency range), options both more expensive and less expensive.

I still have my 6XX, right next to me. Sometimes I'll put them on, give them a listen to see if there's anything I'm actually missing about them, but all it ends up doing is make me appreciate how much better almost everything else is.