I tried them from my Zune and the treble never dropped off, just sounded more edgy. Bass was also slightly bloated when under powered. The K601 isn't that hard to power, at least from my experience. They do benefit from an amp but can get by without one. I don't think they sound all that bad when not amped.
They do improve nicely with an amp. I feel they are OK from something modest, even an E9 drives them nicely.
Weird. Don't think it could be difference in models, since I bought mine from you.
Even compared to a headphone like my CD900ST's (which has a somewhat warm treble), I felt the the K601's were a little light up top. Compared to my K271's, they were miles behind.
Maybe one day I'll own them again, and see if I feel the same way.