: Sweet treble, great mids, non fatiguing, wide sound stage, comfortable
: Nothing at this price range
Both K501 and Sennheiser's HD600 are the sweet points of Head Fi. They are sold at a fair price and they do nothing wrong, they only differ in the way they reproduce music. While the HD600 is soft, smooth and mellow, the K501 has more detail, transparency and more extending treble which is never harsh or piercing in comparison to the K701. The mids of the K501 are magical in comparison to the K701 which is a bit recessed. There is a sense of immediacy with the K501 which brings you closer to the music, a feeling I never had with the K701. It's a pity that this headphones is discontinued, I wish AKG would create a similar headphone with better bass extension and detail, better separation of instruments and better imaging and keeping the same tonality of the K501. The K501 sounds more natural than the K701 which I never enjoyed in the 2 years that I owned them.
Update 01/09/12
I acquired a used pair of K701 which has been used for 1000-2000 hours. I really enjoyed this particular pair more than my K501 which I eventually sold to another Head Fier. The K701 I own now has better detail than the K501, more extension in treble and bass and wider sounstage. The K501 has more coherent imaging and more forgiving with badly recorded music than the K701. I hope that helps!