So my HD558s were delivered on time and I was able to test them out a bit today. Here are my first impressions..
Sennheiser HD 558:
+ SUPER comfy! Ear pads are big enough to accommodate my piercings and the material is nice and cool
+ More bass than I expected, on par with my Bose AE2
+ Soundstage is much larger than on the other phones (except the AD700)
+ Removable cord can lock in place
+ Easy mod to make it sound more like a 598 (will try if I keep them)
- 6.3mm plug
- Included 3.5mm adapter is too thick to be practical (high quality at least)
- Clamping is a bit tight, hope it loosens up
- Cups do not fold, not portable
- Bass is adequate but not punchy like the s400, think I can adjust though
- Certain highs have a strange crackle to them, like turning on your VCR (if folks remember what that's like)
Overall I'm very pleased with how they sound right now. I hope that a couple days of burn in will diminish that artifact on the highs, I hope I didn't receive a defective pair. Also, once the clamping loosens up a bit these will easily be the most comfortable pair I have tested so far. The CAL!2 were a great pair of cans but the HD558 offers more of what I am looking for at the cost of some bass. We'll see if I still feel the same after the weekend!