Just wanted to add a few more words after some more time with the D600's. I don't generally believe in burn-in having great effects on the sound, as I only really noticed changes in about 10% of headphones that I've owned, but with D600's the changes are quite apparent. When I first wrote the impressions they had about 10-12 hours of play on them. Now they have about 50 hours, and out of those 50 I haven't put them on my head for about a week until today, leaving them to play music while they were in their box every day for a few hours at relatively high volumes. So it's definitely not the case of me getting adjusted to their sound or the earpads conforming to my ears resulting in a better seal. It seems like their sound has changed quite a bit. The bass seems to be better extended than before and overall the sound just seems smoother, faster and more tight so to speak. Also, sibilance is reduced greatly, I have several songs where it was a big problem before and now it's nowhere near as bad. Vocals also seem fuller now and the overall tonality is more correct. So, something did definitely happen to them.
One more thing, I think I prefer the way they sound straight out of the iPhone 4S than out of the Musical Fidelity M1 stack. Before they sounded thinner and bass was lacking out of the iPhone compared to M1 stack, but now they just seem to be tighter, especially in the bass, which is now quite bloated on the M1HPAP amp, but also more powerful and deeper. Less bass on the iPhone actually does a favor to these cans and makes them sound more neutral. In the rest of the range, I hear no difference between the two setups playing the same music. In fact, I hear no difference between the M1DAC and iPhone 4S when both used as sources and playing trough the two line inputs on the M1HPAP. I can start the same song on the PC playing trough the DAC and on the iPhone 4S, and then switch between the inputs on the M1HPAP, and I swear even with my eyes closed, I cannot hear any significant difference. It is almost as if the music doesn't stop playing at all, just a tiny silent click and it continues the same as before. I'm not sure, might be placebo, but the M1 might have a tiiiiiiiiny amount more bass and slightly more "dark" tone to the sound, but again, difference is so small that it might as well be placebo, and it's definitely not the case of which sounds better, they just sound a hair different. In terms of dynamics, extension, detail, identical. I compared many tracks at very specific parts where some really tiny details pop out, and on both they were equally presented.
Right now, the only major complaint I have about the D600 is their bass. It still sounds a bit confused. It's definitely not a bass heavy headphone, but the bass has a resonance to it, a certain amount of uncontrolled boom, and that is definitely because of the ear cups. I think its just not damped properly on the inside, and quite frankly, it still ruins the entire headphone when playing any bass heavy music. Also, on electronic music, during the hard hitting tight punchy bass, bass seems to sound like it's going trough some tube, like some echo-effect is added to it, while on other headphones it sounds perfectly clear and tight, even on my DT770's 80 ohm version which is more bass heavy. Basically they can't do fast and punchy bass properly.
That's ultimately what removes these headphones from my ultimate favorites list. Even though the headphones are VERY clear, refined and detailed sounding, with great separation, very close to the Beyerdynamic T1's in those areas (as much as I hate to admit that), and definitely above the likes of HD650 and DT880/DT770, it fails to be a top of the line headphones because of its bass. If the bass was reduced by 10%, and tightness increased to a level of a say DT880, then these would for me absolutely be worth their original MSRP of 500 dollars, and would be the best sounding closed back headphones at that price. Now, they're sort of amazing in some ways, and poor in others. Their sound resolution, refinement, detail is great, simply put, in terms of technicalities these are 500 dollar league headphones, in fact, I'd say out of my entire collection, the only headphones that are better in terms of technicalities are the T1's, and only by a really small, way too small margin. But the acoustics of the earcup enclosure have to be improved, a headphone of this level should not be ruined simply by some nasty reverberation of low frequencies because the manufacturer was too cheap to put a few dollars worth of dampening material into the earcup. This way, these headphones for me work best for acoustic type of music, preferably live recordings where their really amazing soundstage for closed headphones and great depth and imaging can shine. Albums such as Eric Clapton - Unplugged or Antonio Forcione Quartet - In Concert sound best on these, I dare say more involving and fun than on the T1's, although that's only my personal preference. It's just a shame that because of their bass they can sound mediocre with any bass driven music music. That's why I say they're confusing headphones. With certain music I prefer them over the T1's (even with beyers plugged into a 1600 dollar setup, and denons out of an iphone), with other they sound like a 100 dollar headphone.
