I don't really understand why would you want neutral linear response from you headphones. If you are sound engineer that needs to hear details while production you would, but when you want to enjoy music I would say you really don't, and here is my explanation of why. Imagine that you are listening to artists standing in front of you and playing their instruments. What will influence the way you hear them is your distance from them, and with distance higher frequencies are attenuated quicker than low frequencies. Second thing would be fact, that even if an artist stood on your right, you would still hear him with your left ear. Also a room in which you are does make difference, because sound will spread around.
So now headphones with neutral sound, and with no cross-feed may give you ability to disassemble the music into pieces, but certainly it is not the way you should be listening to the music (at least IMHO). The way you should hear the music from your headphones should rather resemble the way you would hear it if it was played live in front of you.
Currently I own four Senns: CX300-II, HD570, HD215, and Momentum. Out of these four personally I love most Momentum. The HD215 made me abandon everything that is lower than CD quality, because I could hear in them every quantization noise, and missing phase information. I fully agree with HD215 review here, the mids are almost clinical and they lack in quantity (not quality) of bass. I used HD570 for a long time, and I considered them very neutral. However I got that feeling the treble were too much emphasized. What is funny with HD570 I never complained about quality of mp3. And the CX300 are an example of bassy headphones, with very unclear treble with a lot of annoying sibilance (do I use correct word for the effect when sound that should be sharp - like a snare - sounds shishy?).
Now time for the Momentum. I fully agree with the very first post in this topic. One who likes to hear bass will like them, and one who like unnatural clinical sound won't. As I say clinical sound is good for production, but when I want to listen to the music an relax, I want it to be an experience and Momentum does deliver. They are addictive very much. They are not comfortable if you have bigger ears (like me), or if you wear glasses (like me). But they are as addictive as Heroine - you don't really care how much pain you need to go though, you just need another dose...
I used N900 as DAC/Amp, and Rockbox player with crossfeed turned on and set to custom, since I prefer to simulate stereo speakers standing in a decent room, rather than having sound inside my head.
The Momentum was for me one step ahead in sound quality, and I am planning to get in some future either HD800, LCD2 or not sure what else. I wonder if there is someone who already has one of these and also has Momentum, and could tell me if for example LCD2 could cure me from Momentum or not?