I wanted to thank all the guys that told me to keep listening to the HE 400. I probably need some time to get use to the sound of new headphones.
-The vocals are much less intimate than the ones on the Senn HD 598, sometimes they even have less extension but overall have a better clarity and imaging and stay on focus even in songs with a lot of instruments and fast sound (The Senn have a more warm and laid back Vocals but sometimes they tend to "fuse" with other sounds, making them "confusing"). Vocals apart, the mids have a nice clarity (more than the HD 598) but some instrument like cymbals, flute, violin, cello and more seam to lack a bit in extension (maybe due to the high mids? I don't know but the details are still there).
-As I said before I only listened to the HE-400 with the velour pads and I didn't notice any echo sound or harsh High that a lot of people are hearing (maybe I'm not prone or sensitive to these effects) even though Highs are really presents and if you turn up the volume sometimes they are a bit annoying but nothing major.
-The bass in the HE 400 is neutral overall (somebody describes them as boomy, dark or even muddy), I wouldn't describe their bass in any of this way, first is not boomy 'cause it simply don't kick hard unless you turn up the volume (than you start to feel a nice kick still don't imagine an overpowered bass) but than you would hear highs and mids too present and unless you like bright listening session you wouldn't find yourself often going over 80 db, second is not dark because It never invade the mid-range or obscure the sound at all, third it's not muddy because as I wrote before it never invades the mids, it never covers other sounds and it's FAST and CLEAR.
- The sound stage is somewhat weird, in certain recordings it defeats the HD 598 and in different one it doesn't. Overall it's pretty decent and thanks to the imaging and detail I think it's a step above from the HD 598. A lot of people are lamenting about a "tunnel" effect and in mine headphone is absolutely not present (fortunately), in my very little experience with equalization and sound tweaking using software I accused a tunnel effect (not with the HE 400) when messing up with the bass and mid range by tuning them up, so maybe you could solve this problem with some eq settings.
The HE 400 are very responsive to equalization so don't tune up them too much or you'll start to hear a lot of artifacts and unnatural sounds (under-water effect, woo sounds, and stuff like that). I tried a few foobar presets that i found in this thread but they always made the sound awkward and I wasn't able to try any of the electri-q presets because every time I try to load a preset with the electri-q foobar plugin it crushes.
The HE 400 respond very well with dolby or THX tuning via software, improving dramatically their sound stage and making the vocals more present and warm without making them sound weird. If you want to watch a blue-ray the HE 400 are much better than the HD 598. If you try to apply any of those effects to the HD 598 you'll hear a lot of artifact in the vocals and a weird sound stage (almost like the vocals are filtered using a can).
-Comfort wise the HE-400 don't excel but are not terrible either. The weight is tolerable (I can wear them for several hours without any pain), I only felt they were a bit annoying just once, after 5 hours no stop of wearing them (and before I had a terrible workout so I was already sore and tired) so if you don't have a weak neck you shouldn't mind the weight. Clamping force is decent but not annoying and if it was less then the headphones will probably "fell on your ear" due to their weight. Most of the discomfort I feel is because of their velour pads (I didn't try the pleather ear pads), the pads are a bit tiny for my ear and the velour itself is made of some sort of synthetic material that don't breath at all so if you have a sensitive skin (e.g. if you are a woman or if you are prone to irritations or dermatitis) I suggest you to find some replacement.
If anybody knows some ear pads compatible with the HE 400 please let me know. I read somewhere that there are some denon pads compatible but I'd prefer velour pads for the comfort. I'm not a DIY guy so I can't make them, I can barely swap ear pads XD.
I don't want people reading my posts getting the impression that the 400's are "terrible tunnel y headphones argh!!!"
