I used my modded HE400's as a reference. After mods, they sound fairly similar. The 400 still has better subbass extension but the mk3 is so much tighter than before. The 400 is certainly brighter up top but I am still able to hear subtleties in the songs I used as references in the mk3 (Hotel California, Get Lucky, Money for Nothing, a few others). Overall, the 400 has a lot better air / instrument separation and better subbass extension / impact down low. However, I like the mk3 so much so for the price that letting the 400 go to clear up funds for Christmas presents won't be as hard as I thought. I apologize if I seem kind of amateur at this, as I pretty much am. I went by my ears alone with no measurement rig so take my impressions with a grain of salt.