Yup, the DSP 24 has a lot of kool features. Would like to see a USB version with similar features.
I was fairly concerned with the 2i2 given some reports about being under-powered, but so far so good. I use it for my measurements (dual 48V phantom stuff included) and seems to have a decent DAC as well.
Anyhow... HD600 phones.
SoundRelative to my HD558, IMO the HD600 are cleaner sounding in the bass region and more balanced. Percussion instruments are better separated and detail is better extracted through the HD600s. Dynamics are very well rendered.
I feel the HD600 has better treble as well. To my ears, the HD600 bass hits a little harder than the HD558. Neither can has wonky midrange or "hollowness" to it. Neither seems ear drilling "fun" or fatiguing.
I like the HD600 better than my HD558, and what I'm listening backs up my previous positive impression of these classic cans.
Differences do exist to my ears. However, to me the HD558 and HD600 are fairly similar sounding. Differences here are not like going from a DT-990 to a KSC-75, HD202, HD558, Neckband or whatever. These cans definitively seem to belong to the same "family".
The HD600 is definitively a whole lot of delicious win.
ComfortThese are very comfy and light. However, while the HD558 drivers seem to be at an angle (or at least the internal felt is), the HD600 seems flat and my left ear barely touches. It's not bad though, and I can definitively wear these for quite sometime and forget about it.
Seems both phones weight about the same.
PresentationA lot of love went into the construction of these. Seems to me the plastic on the HD600 is better quality than the plastic on the HD558, but dunno.
I don't like the HD600 pattern... IMO Senn should have made this phone plain black.
PriceIt seems to me that the lack of worthy competitors have allowed Senn to be creative with the pricing on these cans. One can by a used HD558 for $100... but a used (or "bargain") HD600? $300 (on a good day). For the committed audiophile this is and should be peanuts though.
OverallVery good performance. Definitively NOT

.
Measurements