Don't want to open my DT880 again to double-check, but I read about a bunch of dampening material on the driver inside that the DT990 didn't have.

I don't specifically recall if I removed any white padding, but I did remove a lot of those dampening materials. I felt the DT880 sounded overly dampened to get its more neutral tonality. Similar to how Dan Clark's Aeon Flow can tame the treble with the felt filters, but it crushes detail in the process.

Removing the dampening materials made it sound in-between a DT880 and 990. Not quite as bright as the 990 and a bit more linear bass response than the 990's big midbass hump. Less "soft-sounding" than the 880.

However, I didn't feel like the soundstage changed a whole lot, not as big as the 990.

Check out the European retailer Thomann. Even with international shipping and duties, certain headphones like the K701 end up costing way less because of how high they get marked up elsewhere.

One of my packages came with a piece of foam that has holes punched into it, so I just keep the tubes in there. Extra ones just stay in the boxes they came with.

Good luck with your incoming HD58X! I found the HD58X to have richer, more engaging mids than the Sundara, but I did miss the extension and clean, textured quality of the Sundara, particularly in the bass. I think after hearing both, you'll be able to further narrow down what components you want in your "perfect" sound.