Well, basically, I did the following obvious mods:
1) Purrin's mod... with Dynamat. Do the same thing purrin did, but cover more of the wood cups and also put Dynamat on both the back of the driver and the inside of the plastic enclosure that holds the drivers. Basically, I covered the entire back side of the drivers in Dynamat. Due caution should be taken... as you can see that you're messing with the drivers directly.
2) L3000 ear pads... as you probably know.
And then these... but they are OPTIONAL:
1) A thin strip of foam stuffed inside the ear pads to puff it up. Effect: increases isolation, decreases "some" treble extension (doesn't reduce sibilance), decreases some details, increases bass quantity (quality not affected), increases reverb so it sort of adds a "chamber" effect to soundstage.
2) Felt pads in front of the drivers. You can secure the felt pad by either thin strips of tape, or... by putting a small piece of dynamat in the plastic region directly in front of the drivers. Effect: attenuates high frequencies, so depending on the thickness of the felt, or the shape you cut it out (it doesn't have to be circular), you'll achieve more midrange. Overdoing this may cause congestion, so handle with care... or not at all (preserve clarity)
3) Use some tape and a circular piece of paper to... compress the foam ring that's around the drivers in the front. Just take off the ear pads and you'll see that foam ring. The piece of paper should be circular like the foam ring, and should help cover the foam ring so that tape does not come into contact with the foam ring and damage it. Please take extreme caution NOT to get tape on the paper layer around there. Seriously, this is one of the harder mods, that's why I took it off overall when I sold my W1000X because I didn't want to risk having the buyer damage either the foam ring or the paper layer around there unintentionally. Effect: less treble extension and soundstage width, but a good deal more midrange and a good deal more details... plus more depth. I'm not sure why Audio Technica put this foam ring there. Having the drivers closer to your ears would actually give you a better sound in this case. I was inclined to take it off completely at some point, but I didn't want to do any mod that's irreversible.
4) Stuff the bass ports (near screwholes) up with tissue. Effect: tighter fit with the screws, less "boomy" bass, bass becomes flatter and more one-dimensional, but in exchange, midrange is clearer and also less resonance. If dynamat seems like too much trouble to put on (or too expensive), then doing this will achieve sort of the same effect. Downside is bass. But if you don't care about that to begin with... I didn't attempt this without taking the ear cups off, but I think it "may" be possible that you can stuff those small tissue pieces into those holes just the same.
5) Felt rings in the back of the drivers to cover the space left behind after the yellow fiberglass is compressed by the plastic enclosure. Effect: cleaner bass and less resoance. Also one of those mods that you can do in case Dynamat is too much trouble to put on... though cutting all those felt rings (I stuffed in like 10 rings per cup... so I had to cut 20 rings in total) is tedious all the same.
6) Stuff up that opening on top of the plastic frame near the hinge where you hook the ear pads on. Effect: greatly decreases air, so treble extension isn't quite so "peaky", but sibilance is still there. This one is probably for those who are looking to have a darker sound while not taking away the clarity of the L3000 ear pads. I liked my treble, so I ultimately didn't pay that much attention to this one, but I suspect you can cover this opening in some unique ways so that it doesn't decrease treble.
7) Tissue in the "trench" of the plastic housing where the ear pads are "padded" in the back of your neck. Effect: may be placebo, but I feel that this makes the sound more "solid".
And... that's all I tried.
I didn't try anything too elaborate.