I own the HE-400s, would the HE-500s be an upgrade? Listen to every genre of music under the sun.
More forward mids, more mids detail, overall more transparent and true to the music, but less "clear" sounding (HE400 was tuned in such a way that detail is laid out naked, HE500 sounds a bit more integrated and details that are supposed to stay back, do stay back). Blacker background. Better bass and treble extension. Slightly looser bass (surprising actually, HE400s > HE500 in this regard). Much much less fatiguing treble.
I started with the he400...went to the he500...then back to the he400. These headphone do sound more alike than different but the bass on the he400 is what will win most over. It's an addcitive and fun quality in a hp that's considerably less expensive and much easier to amp properly.
Just got myself the HE-500. If I am in a passive mood and want music to waft over me, its the 500. If I want "aggression" and a pick-me upper, its the 400.