I'm going to try to keep everything short and to the point.
Background : I am a heavy classical listener, attend numerous concerts throughout the year, and listen to classical music pretty much single day for hours.
My head fi experience - I got into headphones few months ago, and I have gone through the following headphones : HD650, HD558, KRK KNS 6400, KRK KNS 8400, Panasonic HTF 600, DJ100.
What headfi did to me : Ever since joining head fi, I've been thinking about headphones when I attended concerts. After the most recent 2 concerts within the last week and a half (SF Symphony playing brandenburg concertos by bach, and New York Philharmonic last night playing tchaikovsky 4th and Bronfman playing beethoven piano concerto 3 with dvorak's overture as an opener), I strongly agree that my KRK KNS 8400s are as neutral and true to the recording as they get for my budget range headphone ranges.
The only complaint with the KNS 8400s is the closed headphone, so there isn't a huge sense of 3 dimensional sound space, but nevertheless, they are the closest out of any of the headphones out there.
Again, these are everything that I heard and is in my OWN experiences and opinions, so I'm not claiming that this is 100% true, but I feel that I have a stronger grasp of music then the average person.
The verdict :
I was worried before purchasing the 8400s because people were complaining about harsh treble and lack of bass, but THEY LIE! (bold claim, I know. Ha.) I don't want to go into too much details, but I feel like the 8400s produce them exactly as they do, and that's what I wanted. Also, as everyone else said about these, the details and separation, the accuracy of the soundstage is all there. I tried to find flaws in it, but I could not.
The Sennheisers seriously have a major veil to them that I have a hard time listening to. The details just aren't there, and the trebles not represented accurately and too recessed. I believe Sennheiser was going for that, but it doesn't match what I'm seeking. If anyone wants an accurate representation of a classical concert, I would NOT recommend these to anyone; however, they are VERY comfortable, and I absolutely love them for enjoyment when I listen to Muse. The soundstage on both 558s and 650s were very nice, and I can wear these headphones for hours.
I didn't do a full review on the KRK KNS 8400s because there are plenty of reviews by other people on them, but I just wanted to share with head fi my thoughts on neutrality and headphones, and what I believe to be an honest and neutral headphone at this budget range. At this budget range of around 100 dollars, I don't think you'll find any headphones for neutrality as good as the KNS 8400s. I may get around trying to get a HM5 for review, but I'm really satisfied with the KNS 8400s for what I'm looking for in music for closed headphones.
I am currently thinking of replacing my open headphone with the AKG K702, but I'm still doing my research on it, and I heard it's uncomfortable which is a shame. I'm sure I can find a workaround if the sound is that great.
I'm currently listening to Tchaikovsky's 4th with my KNS 8400s, and I feel as if I was sitting in that concert hall last night minus the soundstage. ON A SIDE NOTE, that was one the most amazing concerts in my lifetime. It's my 2nd concert that I know I will NEVER forget, and I also got a picture with Maestro Gilbert which is absolutely amazing. Anyways, asides from the open headphone choice, I also plan on building my own speakers over summer, so I look forward to getting into speaker building as well. Well, this ends my thoughts on my current set of headphones and the term neutrality!
FINAL WORDS : Neutrality is not for everyone. Almost everyone that's not experienced with high quality music found my KNS 8400s extremely boring and unengaging to listen to, and I totally understand. Get something that you'll enjoy your music with, and forget what everyone else tells you. As long as you're happy with the sound, that's all that matters. For me, my "fun" headphones are headphones that replicate the recording in a true fashion, so find the right headphones for your genre of music and your preferences in sound. I tried to keep this neat and simple, but I gave up half way through. I see why everyone posts such a long block thought now, haha.