I believe that DT880 and HD650 sound better unamped than HE-500 does, because they have much higher sensitivities - 96db for DT880 and 102 db for HD650 vs. just 89 db for HE-500. If I am not mistaken, sensitivity matters more than impedance in determining how hard a headphone is to drive. Also, I read that higher impedance is actually easier load for many amps. From personal experience, I find HD650 and DT880/600 more focused, spacious and dynamic sounding than HE-500 out of weak, consumer grade portable sources. Out of something decent like an E11 or HM-601, HE-500 improves greatly and begins to compete with the other two cans in technicalities. Out of something like STX, HE-500 starts surpassing HD650 and DT880 overall IMO.
Thx! I agree about sensitivity rating on HPs. Just like speakers, higher sensitivity will get more dB.
In my case, the HE500 sounds better out of portables than my Q701.