Alright. Went down to the headphone store today and listened to my HE-4, some HE-500, HE-400i and also some Alpha Dogs.
First of all, I listened to the Alpha Dogs right before I packed up as I didn't think closed headphones would do it for me. But wow are they impressive, bigger soundstage than the HE-400i I think! Roughly the same mid-high treble level to me as my HE-4 but it definitely has more bass and smoother and more body to the mids (though I wouldn't say they are any more forward than on the HE-4). They are very comfortable and when listening I really didn't think I was listening to closed headphones.
Anyway, onto the Hifimans. First off, the differences between the HE-4, HE-400i and HE-500 are far from night and day to my ears.
Summary
HE-4: Large soundstage, great imaging, great attack, quite a lot of life to higher notes on guitars and drums sound fantastic
HE-500: Relatively large soundstage (smaller than the HE-4 but I feel instruments blend in well together whereas HE-4 leaves large gaps between each), very smooth and relaxing mids (especially male vocals; can sometimes be boring), highs aren't overwhelming like HE-4 can be but are still prominent
HE-400i: Small soundstage, good amount of bass but strange low to mid-bass which can be fatiguing, vocals sit behind instruments, great impact, fatiguing highs
Overall, the HE-400i definitely are not what I want. They sounded the best out of the lot with poorly mastered recordings which lacked life and detail, but that was it. I found them engaging, however quite fatiguing like some Grado's (to my ears). They also sounded quite congested with some tracks and it's like it failed to play all the bass notes from the bass guitar smoothly and clearly.
The HE-500 weren't as good as I was expecting. For some reason I really enjoyed the LCD-2 but not the HE-500. The vocals on some tracks truly sounded great but not amazing like some describe (could be poor synergy with my amp but regardless I'm not going to get another/different amp). The highs were nice and smooth but I think it's the upper mids which are still too sharp for my liking like the HE-4. They just seem too smooth and relaxed in the mids/lower mids and too edgy later on until we get to the cymbals (from the drum kit).
As for the HE-4, which I of course own, they sound by far the worst with the poorly recorded/mastered songs. The large soundstage made them sound even worse due to the fact all the instruments and even the vocals lacked life completely (probably due to prominence of treble and lack of lower frequencies) which made the sound very flat and lifeless with little engagement. On some tracks it also sounded like there was no sound coming from the centre and only from left and right.
Also worth mentioning that on a scale of 0-100 on my amplifier's volume control, the HE-4 needed to be about 20 higher than the other HifiMAN's.
Overall the HE-400i aren't what I am looking for and the HE-500 weren't exactly what I am after either and thus I don't think it is worth while to go through the hassle of selling the HE-4 and purchasing HE-500 to just be content and not happy/excited about new headphones.
I might go back next week to try out some more headphones.