I mite try this mod, I don't mind taking my brand new cans apart since I play with computers for a living as is. Headphones are no big deal.
I have had my DT990 600ohm premium cans for a couple of months now and I've finally got a really good feel for the sound they put out since they are finally nicely burned in.
Sub bass-mid bass is premium quality "IMO-, In my opinion" it is punchy, clean and up front. Not sloppy at all.
The highs can tend to be a bit bright but with a minor EQ change it is not a problem for me at all. I like the fact that the highs are crisp and clear and not drowned out by anything they come through very well.
Now the mids, oh yeah those mids on the DT 990's fall rite off and get pushed back by the mid bass and sub bass. These cans sound great with R&B, some Rap, movies and games. "for under 400$ Canadian I've never had a better pair of cans for these things'' but for Metal, Hard Rock, Blues, the pushed back mids suck "IMO" I am really not impressed and for me that is a huge factor, the majority of what I listen to could be classified as Hard Rock and Metal and for me not having a nice chunky mid for the guitar, vocals and part of the drums and bass is a real pain in the ass.
This is just my opinion on the DT990 600ohm Premium cans, I know to some people it may sound like I'm being a little harsh on them for the mids falling off but it is just my opinion. I really like a strong mid sounding headphone so in that case yeah I am a little bit more opinionated about the mids falling off a bit.
Lastly I don't find the bass sloppy at all, I find it very clean and punchy. Possibly just because I have the valour cushions. "I think they probably would absorb a lot of sound, that could make a huge difference in the bass compared to the leather cushions.
I like your mod idea and the directions you put up here are great, I will try this when I get the time and post some pictures of before, during and after.