drwlf
500+ Head-Fier
The sound depends on what the person who built it did such as how the drivers are mounted (tape or press fit), if they were flush or stuck out a touch, dynamat and how much was put in the cups, ear pads used (L or G pad), and also it could have been a defective driver (I had to send one pair back since it was as you say it sounded) and as always its a personal preference and how we all hear differently. Had a friend who bought my HD-600 take one listen to the Ypsilon and the HD-600 was history the next week and she built her own Ypsilon build, a few other head-fier who have heard my pair liked it more than their Ether, Ether C, HD-650 and LCD-3 so I think the one you listened to probably sounds quite different from mine.
Would be interesting to hear a pair!
And related to the thinness discussion above and in general, I wouldn't exactly call 400i a "thick" headphone