I've been researching headphones for the past month and decided to jump on the HD558 for $80 deal at Best Buy. I figured they would be a good first headphone that didn't break the bank.
Warm overall signature, good mids and midbass, but the treble rolled off. I figured I'd give the black foam mod a try and it only took a few minutes to remove it from each cup. That opened up the treble but gave also gave it a slightly rough edge, most noticeably in vocals.
Well, instead of complete removal, why not reduce the amount of foam? I considered cutting the black foam but decided against in case I wanted to reverse the mod.
While at a craft store, I found a single sheet of adhesive-backed foam in the kid's section for the grand cost of 99 cents. Black was all gone so I picked white (the red foam sheets were covered in glitter). Not like anyone was going to see it anyway. The foam was the same thickness as the Sennheiser foam but a little stiffer and firmer.
Now the question is how much foam should I use? There's only a bit of harshness in the treble so I decided to go small. I cut two 1/4 inch wide strips of foam, checked if they were centered correctly, and pressed them into the cups.
That seemed to do the trick, it smoothed out the rough edge I was hearing in the treble. Clarity, detail remained.
I re-opened the cups and removed the 1/4" foam strips so I could A-B a few songs to make sure it wasn't my imagination. It's really a slight difference and some may not notice at all. But if anyone does hear that bit of harshness, I recommend trying a 1/4" foam strip.
Edit: Added image.