I like to chime in my appreciation for the 6400, I don't have 8400 so I can't really comment. I've got it plugged into a Yamaha AVR and let it absorbed all kinds of stuff from HD radios for well over a week! When I first cracked open the box and listened to a few tracks of my favorites, I couldn't hear some of the instrumental separations (soundstage) everyone was talking about - and I'm not so sure I can either after well over 7 days of continuous burned-in. Also the lower frequencies sounded muted and subdued, more so than I thought. Other than that, clarity in the highs and mids are awesome -- though mids sound somewhat lack of robustness, but clear.
Perhaps my ears are tuned to less stellar cans, akin to K81DJ. For some odd reason, I enjoy my re-cabled K81DJ more than I enjoy 6400. Yes, I removed the foam pad immediately after I received it. I still have it but contemplate on selling it. I got it for $80US of eBay.