Got mine today - a day early! Listening now while I have a chance. Initial impressions are very good. My chain is FLAC files from PC --> Behringer UCA202 --> Garage 1217 Project Sunrise 2 (stock tube).
DISCLAIMER: I don't have "golden ears", I don't typically listen "critically" to music and I sure as hell don't know all the lingo. But what I do know is what I like and what I don't - so I'll talk about that here. Take this as one man's ramblings and that is it.
My other headphones are in my profile, but I will list them here as well as a quick rundown on my thoughts on them:
DT880 Pro (250 ohm) - I keep going back to these as my reference time and time again. I love these headphones. They are not too bright to my ears and I believe that the bass, while lacking when the pads are new, gets really good when they settle in a little closer to your ears.
HE400i - tough to say these are 2nd to the Beyers, as I'm not 100% sure that they are. The Beyers do classical music so well and are so comfortable that they still get much head time, but these have an excellent sound even if a little rolled off in the highs. Plus, I just love the bass and midrange of these HifiMans.
HE400 - these have the Jergpads on them. They are OK IMO. Not great. These were my first planars and from the moment I got them I wasn't really sold on them. I rarely listen to them anymore as the 400i has just as good of bass IMO and does literally everything else better.
Massdrop HifiMan RE00 - I use these when I workout and if I need a portable decent headphone. They are great for that. Nothing really stands out to me with these, but then again, I've not spent that much time listening to them.
Bose Quiet Comfort In Ears - I use these when I travel or when I'm playing my electric drum set. They are not used for any enjoyable listening. These are amazing headphones for listening or watching movies while flying though!
OK, so on to the HD6xx:
I wasn't sure I was going to be sold on these at all, but I'd heard so much about them over the years that I couldn't hold back from picking them up at this price. I've always wanted to try them, but the idea of the Sennheiser house sound - rolled off highs and the fabled "Veil" always held me back. My favorite headphone being the DT880's should be a tip off that I like a more "neutral" if not "bright" headphone. But hey, for $199 and free shipping, I would have been crazy not to dive in and give the Senn house sound a shot. Besides, like most of you, I love listening to different sounds and experiencing music in new ways. I've always been told that the best way to change the sound you hear it to change headphones first and foremost. I agree.
The Sound - Anyways, listening to some Ryan Adams Live at Das Haus in Ludwigshafen, Germany and I am blown away by how "musical" these headphones are. I don't know about synergy with my setup or whatever, but these seem to pair well. There is definitely less high-end treble energy than the Beyers, but I'm not getting the feeling that I'm missing anything in the music. Maybe that's because the HD6xx's do the mid-range so well. The bass is just enough. Similar in quantity to the he400i, but definitely not as controlled (no surprise as the he400i is seriously tight when it comes to bass). The HD6xx definitely have more bass than the Beyers though, that's for sure, and it makes the sound kind of "fun" but without detracting from the mid-range. I think this is a sound that I will enjoy for years.
The Build Quality and Clamp - I have a pretty narrow head and I've been wearing these for over an hour with no issues from clamp. Actually, now that I'm focusing on it, they are a little tight, but not that bad. Generally, they definitely don't feel as well made as my Beyers, but I've read that nothing really ever will. Those things seem to float on my head and I can wear them forever and not really care. These feel light enough on my head for longer listening and I'm sure when they aren't quite as tight, they will be fine. If you have a bigger (wider) head, these are going to be a little uncomfortable.
The Cable - It's definitely shorter than advertised, but I really don't care. I use these at my desk while reading/playing chess, etc. so I don't need a longer cord. I'd also like to say that I was pleased with the lack of microphonics this cord produces. Especially coming from the RE00's. Those are a disaster for a portable IEM.
I'll update this as I get the chance to listen to my other headphones and more music. For now, I'm really quite pleased with my HD6xx's!