Meet impressions:
HE-500
Very comfortable, though really heavy. Sound is very good. Mids feel a bit absent, but overall the sound is quite balanced. Bass is deep with nice impact, and has a good soundstage. This thing is wow. If it wouldn't cost so much, I'd definitely want one.
HE-6
Dang, this thing needs gobs and gobs of power. Actually the 4W per channel Burson had trouble driving it to even normal listening volume.
Sound is very smooth, and I think I like it a bit more than the HE-500, but definitely not worth the price gap. Wow.
Did u try both the he500 and he6 on the 337? or just with the burson? As for instance the mids on the he500 was full and velvetly on the 337...Most who did try that were surprised how good it sounded with both phones..even some tried the audezes on the 337 and as far i saw the smile in their faces.i think they liked what they heared