[AutumnCrown]

The  400i was my first audiophile grade headset, and i really enjoy them. At some point, I may choose to get another pair so my 400i is less lonely I would look for something that is a nice contrast to the 400i. What I like about the 400i is the realism, detail, smoothness, speed, and flexibility. What I think is lacking from it is a certain lightness/brightness and treble edge that makes sparse acoustic guitar tracks really sing. My favorite musician being Nick Drake, this is a priority. However, I don't want to compromise too much on detail and realism.
 
What I've considered:
 
Sennheiser 600: I auditioned a pair of 598s and though they are a class below the 600, I appreciated the soundstage and the smooth Senn sound. 
 
DT 880:  often mentioned as a great value can for treble heads, without being too difficult to drive or abrasive in the way that Grados are often described as being.
 
Grado ?: I don't know much about which model or line would be a good compliment for the 400i, but I know they tend to be bright!
 
Senn HD800/HD800S: If I wait until I have a bit more financial stability, I might consider something in this price range, as I understand that it is pretty great at everything and rather strong (perhaps too much) in the treble area.

Any other suggestions?

 

[maul]

  The  400i was my first audiophile grade headset, and i really enjoy them. At some point, I may choose to get another pair so my 400i is less lonely I would look for something that is a nice contrast to the 400i. What I like about the 400i is the realism, detail, smoothness, speed, and flexibility. What I think is lacking from it is a certain lightness/brightness and treble edge that makes sparse acoustic guitar tracks really sing. My favorite musician being Nick Drake,  so this is a priority. However, I don't want to compromise to much on detail and realism.
 
What I've considered:
 
Sennheiser 600: I auditioned a pair of 598s and though they are a class below the 600, I appreciated the soundstage and the smooth Senn sound. 
 
DT 880:  often mentioned as a great value can for treble heads, without being too difficult to drive or abrasive in the way that Grados are often described as being.
 
Grado ?: I don't know much about which model or line would be a good compliment for the 400i, but I know they tend to be bright!
 
Senn HD800/HD800S: If I wait until I have a bit more financial stability, I might consider something in this price range, as I understand that it is pretty great at everything and rather strong (perhaps too much) in the treble area.

Any other suggestions?

 
Wow, you think the 400i is lacking in treble? Not a common opinion, they are certainly not even remotely dark. Maybe DT 990s is what you want, if you are into the whole ear bleeding thing, heh...

 

[AutumnCrown]

Not so much lacking in treble in general, just in certain darker tracks. In fact, it may not be so much lacking in treble in these tracks as much as having overly forward vocals which makes delicate guitar, for example, difficult to hear.

 

[marhol]

I think HD 800 would be a very good complement/contrast headphone, especially if you are into treble detail. HD 800 has plenty of  (treble) detail and besides of it its soundstage and imaging are second to none.

 

[cskippy]

I would have to recommend either a DT880 or HD650. The HE-400i has some pretty severe dips at 2kHz as well as 6kHz. There is also a sharp peak at 9kHz. This makes them sound bright with some recordings and dark with others since the mids are not smooth and less present overall. Hd650 is the king of mids and a great all arounder. Modding them take them to another level. Plus dynamics present music in a different, maybe more natural way, IMO.

 

[AutumnCrown]

I tried the HD800 at a store. This was foolish, because now I know that I must have it, despite the price, though probably not for at least a year : /
 
EDIT: lol a year! yeah right, I lasted less than 2 months.