If anyone has done it before, when EQ tweaking what frequencies should I be reducing and by how much to "fix" the treble bite of the 880's?

The 3mm thick wool felt mod, will the black foam be completely removed and replaced with a similar sized piece of felt? How is that plastic ring that holds the foam on prised off, a flat screwdriver wedged in between the ring and the frame and gently prised off?

As luck would have it I found a piece of 3mm felt around the house and put the felt over the black foam, does seem to help make the trebles a lot more palatable.

Apparently you can make an 880 sound like a 990 quiet easily but going the other way is not quite as simple.

You can't make an 880 sound like a 990 nor an 990 to sound like an 880.
They differ far too much.

Yes, just put a small screwdriver between the ring and the cup and 'pop' the ring out.
This is the same for DT770/880/990.
Be carefull NOT to ram your screwdriver through the paper filters under the foam.
Once the foam is out you cannot put the felt under the ring as it is too thick and you won't be able to fit the ring.
The ring 'secures' the driver baffle so the ring must be put back.
The felt needs to be the diameter of the inner side of the ring so it 'fits' inside it.

What you could try (to secure the felt) is to make a ring and have 8 (or so) small 'tabs' that stick out beyond the felt ring and squash those 'tabs' under the ring when putting it back. (just an idea, did not try it.... yet)

On older Beyers there is no ring with foam and you need to 'pop out' the baffle to get inside the cups.

Below the effect of 3mm wool felt in front of the driver.
I don't care if anyone thinks the measurements are 'incorrect' in an absolute sense, they may well be.
All they do here is show the effect the felt has.
Needless to say blue = stock DT990Pro 250 green is with felt.
I saw little to no change in the CSD which is more than decent anyway.
The bass also seems to improve by the extra damping a bit... didn't hear that though.

Tried 2 pieces of 3mm felt at the same time and the 1/4" rope under the earpads to lift the ear further from the driver, still can't tame the painful piercing at anything even remotely close to normal listening levels.

Apart from the trebles that are as harsh to my ears as anything I have ever heard hi-fi wise they sound fine albeit a touch anaemic(which is supposed to be the expected sound signature). The trebles sound to me like a cheap hi-fi speaker's tweeter when overdriven/clipping, really nasty.

The headphones are open box, perhaps the drivers are damaged?
How would damaged drivers sound or how would I test for that?

As you can see only 5dB of treble nasties is removed.
This is quite substantial yet as the plot shows far from the desired levels.
About 5-10dB remain.
The felt does improve the treble.

In case of the DT880 you will thus probably only 'shave off' some of the treble.
The bass won't improve of the DT880 nor will it become warmer nor less 'analytic' just some of the harsh treble is lowered.
For some it is enough, for others too much and for others again not nearly enough.

If you want less midrange and moar bass you could swap for a DT990 BUT the treble will STILL be too much for you.

EQ is what you need OR one of the other headphones that were suggested.
personally I would try to sell the DT880 and buy something that is not piercing and warmer from the raised funds.

I don't think your drivers are damaged, both would have to be and the sound you describe is about 'right on the money' for a DT880.
Some really LOVE that sound and call it flat and analytic/detailed, others hate it and call it bass-shy and piercing.
Its the reason why there are so many headphones around which are liked by some  and hated by others.

I'm really struggling to understand how my ears can't handle the sound signature of these headphones, failing the drivers being somehow damaged?

If I turn them up to normal levels they make me feel physically ill. My main speakers are Harbeth mini monitors, modern equivalents of the famous LS3/5a BBC monitor speakers, which are renowned for being amongst the most revealing of speakers, especially in the trebles yet the 880's with a rather similar sound are unbearable.

I don't know how to account for it.

http://www.innerfidelity.com/images/BeyerdynamicDT880250ohm.pdf

Look closely at the 300Hz square wave. See how the leading edge bounces up and down a little? Headphones that exhibit behaviour like this almost always sound harsh in my experience. The 10kHz peak in the FR definitely isn't going to help either as it's higher in amplitude than the entire remainder of the FR.