This headphone really has it all. It is easily comparable with other headphones costing twice it's price. Shame it doesn't get more love it really deserves here on head-fi. 
 
A highly musical headphone with good bass, open mids and very detailed treble. It will probably earn a legendary status with time for it's unique sound and great comfort. Well done Beyer.

400i is in my world actually different beast to the old HE-400 and should have another name. HE-460 would be more suitable I think. The sound is much more balanced than the old HE-400 which makes it suitable for different stiles of music. The sound of HE-400i is very clean, transparent and life-like. Compared to much more expensive headphones it can get a bit bright and congested in the mids. This is only when compared to planars costing more than double its price. HE-560, the big brother, has a bit clearer (more refined) mids, better soundstage, little less pronounced bass and a tad more treble energy. Overall the sound signature is quite similar. I am not sure if the price difference ($400) is justified in this case. The little sibling HE-400S has a bit less bass extension, messier and less defined mids and a bit more treble energy (although 400i has a bit more detailed treble anyway). The price difference of $200 is not that big but probably worth the "extras" in the sound of 400i. 
HE-400i is a worthy headphone and a nice introduction to orthodynamic headphones.

This headphone is one of the best headphones for classical music. Others in the same price range would be Audio Technica ATH-AD900, Beyer DT880, AKG k701. I think that k501 can even challenge some other dynamic headphones in higher price ranges. It is definately better than Sennheisers offerings HD650/600, for classical. This is a real classic, one of a kind.

As much as I cannot understand the overwhalming positive reviews of these headplones I admit that I actually can understand that people can get in love with their pleasant and forgiving sound. If you wish to get close to the soul of a recording however, you should go somewhere else because HD650 are neither natural or neutral sounding headphones. Compared to some other cans (AKG k70x or Beyerdynamic DT880 namely) these Senns are muddy to say the least and lack sparkle, transparency and sheer excitement. The bass is more prominent than with k701's but the definition of the same is not as good. Listening to the HD650 I really miss the sparkle and the excitement that should be there. When listening to accoustic music and classical there is no air at all. A huge amount of information, upper harmonics are simply absent.  It is important to say that I have had some issues with my pair. One of the drivers broke after three years of use. The paint on the headband started chipping after 2 years.
 
However I can understand that many people get in love with their sound. They sound "relaxing" in their own way and are very forgiving on the quality of the recording. The problem is that you can get all that and more at half their price.
 
Amplifiers used: Lehmann Black Cube Linear, Musical Fidelity X-Can v3, Matrix M-Stage

These are one of the best headphones I have ever heard, and I have heard a couple. I undoubtedly prefer them to Senn HD650, like them slightly more than Senn HD600 and Beyer DT880. Why? Because they have more natural sounding midrange and airier soundstage, characteristics that fit the music genres I prefer (classical and jazz). In terms of neutrality these are as good as Senn HD800. The bass is nicely extended and precise. Mind you, they need proper amplification. Don't expect them to sound great right out of an iPod. I have heard mine with MF x-can v3 and there is no synergy between the two. A better match is the cheap Pro-ject headphone amp. They sound even better if you drive them with a more powerful amp (ie Burson). I have mine coupled with a Matrix M-Stage V2 and the combination sounds really magnificent. All in all these are one of the best sounding headphones around, and one of the absolutely best values out there today.