HE300s are warmer, more mid-centric. The HE-400s are more leaner in that respect. Bass is tighter, faster, more linear. Actually like it better than my LCD-2's bass at times. The HE-300 I remember not having the lower treble suckout. The HE-400 do have more sparkle up top though. Proficiency-wise, can't compare. These are detail and separation monsters.