oratory1990

Just try it out!
If you have to turn the volume to 100% and it‘s still not loud enough, then you need an amplifier with more power.

The graph on the Qudelix shows the frequency response of the EQ filters that the Qudelix applies to the signal.
It shows you how it changes the signal.

It does not show you how the headphone sounds.

That was in regards to their sound quality, not with general electronic components. Batteries work less well in low temperatures, that has nothing to do with the headphone itself.

This is currently impossible to do with active noise cancelling, and will be very uncomfortable to wear with passive noise reduction.

Are you talking about 70 dB of noise reduction, or reduction of noises which have an SPL of 70 dB?

I don't know, man.

I starte by buying a bunch of cheap headphones for the recording studio (I needed to record a choir, and needed a way for every single singer to hear the background music without it being recorded by the microphone).

Then I bought a few more good sounding headphones (for those musicians that wanted to hear whether the recording sounded good enough).

Then I bought a really good sounding headphone for myself. And another one.

Anyway, I now have a few dozen headphones in the cupboard, another few dozen on my desk, some in my gym-bag...

or do I need to decrease the preamp gain at a point to avoid clipping?

if you hear clipping, decrease the preamp gain (meaning: "a larger negative number").
If you don't hear clipping, you don't need to do anything.
Clipping is not subtle, you can't miss it. If the sound goes "bbrrrzzz", then it's clipping.

You‘d put it in the seams of course, not behind the opaque surface :)

It‘s the job of the mixing engineer (more specifically: the mastering engineer) to not make you want to use different EQ settings for your headphones for different songs.

But since they’re just human, they won‘t perfectly succeed, and it‘s normal to somewhat want to adjust the EQ for different songs, especially if they were not mastered for the same release/same engineer/same album/…

Protip: use Lazerbond or a similar UV-hardening glue.
Super cheap and doesn‘t harm the plastic. Can easily be cleaned.

The answer is probably „because that‘s what the studio bought ten dozen off back when they got monitoring headphones“

Cut it manually from a sheet of polystyrene.