I don't own both now but used to own them together, so can offer a minor comparison from memory. The T90's have a wider soundstage and have the Beyer house sound, (as you own the DT 880 you will be familiar with the sound), crisp & clear. The HE 500 have a more intimate sound stage , its not constricted but its not as wide as the T90. They sound more "organic" than the T90's, more natural, acoustic instruments sound more true to life on the HE 500. T90's will have a slight edge where EDM is concerned, whereas HE 500 will have a slight edge with Rock. When I used to own them T90's were my absolute fav for Electronic & HE 500 for everything else. The T90's are way more comfortable than the HE 500, its like comparing a business class seat to economy.
The T90's can tend to sound a bit bright for the first 150 -200 hours & then the treble mellows down a bit but quite a few people will still find them bright when compared to a other HP's. T90 needs a tube amp (preferably an OTL amp & to narrow it down even further, a Bottlehead Crack) to really sound their best. People have had good results and do like them on solid state amps as well, but once these people hear it on a Btl Hd Crack / OTL amp they will never go back.
HE 500 is not so fussy about amping but they do scale well as you incresae the amp quality. They need an amp which will output atleast 1w @ 50 ohms to shine. I found them amazing on the Meier Classic & Schiit Lyr but not so great on Woo 6 in comparison.
When i was doing my research for buying both I had read both HP threads in full & found that HE 500 are a more safer bet i.e. out of 100 people who buy the HE 500 only 10 will not like the sound whereas with the T90 , generally 20-25 will not like the sound & almost all of these people have issues with the treble (these are very approximate numbers, just to give you a general idea). If you are not planning to change your amp & you cannot audition both, then the HE 500 will be a more safer bet for you.
The above is IMHO & lot of people will have a different opinion.
Thx, You know the build quality of the HE-500's as that also important for me. is How is the imaging is it 3dish or close? As I think about it a couple of times my hand slip and hit the cable that sticks out on the beyers scaring me, as I don't look forward to having that break one day.
Edited by genclaymore - 3/28/14 at 4:56pm