I'm still waiting for a Klein bottle speaker cabinet.

I can't tell you how many times I've mentioned this to headphone makers.

Every speaker in the world has damping, why don't headphones?

Stupid.

Ranks up there with the one size fits all ear cushion. Pet peeves.

I actually think this may go way back to the days when everything was eq'd for am radio.

they still used a form of damping with the baffles cause they were semi open with some type of foam in the outer part of the cups. the drivers too had specially tuned acoustic chambers as well. the pioneer monitor 10's back was made out of leather so it was a form of damping i think besides aesthetic reasons. it also used foam cushion for front of the drivers outside and left the dome alone to allow clear high frequency extension cause i found the monitor 10s probably one of the most neutral sounding closed dynamic headphones personally think and probably has the most extension at both ends compared to a lot of modern headphones that can't reach and usually roll off at.

vintage planers and electrostats used damping as well both mechanically and acoustically. seem the engineers of the professional line for all 3 types of headphones being dynamic,planer, and electrostatic took good deal of their time designing these headphones cause they wanted to be close as possible to like speaker monitoring. most headphones nowadays changed not because tech has but the marketing. it's all about making money always no matter how respected ''that'' company is.

the only way your going to see damping in headphones nowadays is it's either very expensive or it was intensionally put there to try cover up the flaws of the driver itself, not to help it.

Are there any measurements of before and after adding some damping material in the cups and ideally nothing else modified?

IIRC, IF had an article on some HD800 mod but the changes were small.

i see where you're going with this and i do agree. be interesting to see just to see other side of everything see what is what and so forth. i think though overall it will ''depend'' but i know where you're getting at.