As for comparison.
The Fostex TH600 is far easier to drive and can even sound pretty decent out of portable players like the Fiio X3 depending on the songs. On my NFB 5.32 I run it on low gain around 10 o clock, by comparison the high gain at the same area powers the HE-500 to moderate levels. 12 o clock is loud. The NFB 5.32 puts out 2 watts at 50 ohms at full power or something similar.
The Hifiman is far heavier and is less comfortable all round. I found the velours painful with my glasses until I put them in warm water and let them dry off to soften them. The Hifiman sound changes depending on the pads, sounding there best with Jergpads, while the pleathers can increase the bass and lower the treble levels. The result is a darker more "congested" sound. The Velours are much more extended in treble and have a larger soundstage, however the sound to my ears is pretty dry for whatever reason and I found it to sound somewhat unnatural. It could be the amp I guess as well, but none of my other headphones sound dry.
The TH600 is light and incredibly comfortable, they only come with Protein leather pads, but I guess you could try pads that are from other headphones. The cables do not detach so balancing is harder.
The overall sound of the HE-500 seems slightly dark, laid back and to me it is fairly forgiving. The soundstage is reasonably wide but the depth lacks compared to some other top models like the T1 and HD800. The imaging is very good, and is one of its strengths. I rarely feel the sound is cluttered or incoherant. The HE-500 does well with most genres, and is a very good all rounder. I like it a lot for Rock, Metal, Pop and other energetic styles. The mid range is very linear till the upper mid range which has a strange recess, this can be fixed by doing the jerg pad mod. The HE-500 leaks a lot of sound, practically unusable if you do not want to be heard.
The TH600 is a bassier headphone, with powerful and deeply extended sub bass. The impact and rumble are exceptional as is the quality of the bass. It is my 2nd favorite EDM can to my TH900. The soundstage is remarkable for a closed headphone. This is not a case like the Mad Dog where someone goes, oh it sounds open not like a closed phone, but still can be hear as being closed. The TH600 sounds remarkably clear and pitch black with a great sense of space. It can be almost cavernous (in a good way) at times like the TH900. The TH600 and TH900's imaging is the best I've ever heard in a closed headphone, up there with many open headphones. The Mid range is slightly receded but is smooth, guitars are less prominent and vocals slightly so on my set up, but it is only a minor thing. The treble is a bit hit or miss and can sound etched/harsh on some tracks and perfect on others, showing it to be revealing of the source material. The TH600 leaks a fair bit as a closed phone as it is not truly sealed but it certainly does not leak as much as the HE-500.
The main weakness of the HE-500 is the upper mid to treble transition dip which causes a unnatural tonality, needing a jerg pad fix. It also is fairly demanding of an amplifier. The HE-500 is also slightly slow to my ears with some of my most complex music. The planar sound can lack air as well.
The TH600 weaknesses are the treble at times and slightly weaker midrange. You could probably fix this with a tube amp. I also feel that the bass can be a bit too strong with some rock and metal tracks compared to what I personally like, but that is just my preferences.
I hope some of this information helped, sorry that I cannot go into more detail at the moment.