Interesting. I have the HA-M750's and they look exactly like these.
I like the sound of the M750's. It's decent, but I can't wear them for long. Maybe an hour max? My ears start to hurt or get too sweaty.
Hopefully the S500's don't have this problem as that's going to suck.
Also, can someone comment on the sound isolation of the S500's. ie. if I go to the library/silent study and use the S500's will people hear my music?
Comfort in 750 vs 500 depends on - form of your skull. Since 750/770 earcups do not swivel, they introduce sore poin/line in front or behind your ear, if bones of your skull do not match the angle of 750/770 to lie exactly perpendicular/flat to your bones. 500 is long term comfier, particularly using velour pads, which cut too much of bass even for normal people, let alone for bassheads. Tape mod necessary in case of using velour. Please see the general earpad discussion here.
No, if you listen at anything like normal listening levels that do not lead to permanent hearing demage, your fellow librarians should have no idea what you are listening to.