AKG K501

Compared to the Q701, K701, D7000 the K501 spend a very relaxed sound to me.
You could here with this headphone many hours without tiredness.
Compared direct with the Q701 I notice a little rolloff on both ends (treble, bass). 
A very good headphone for little instrumentation ... e.g. a vocalist and a guitar and for some classic music too.
It's a headphone that headphone »freaks« must have.
Cons - AKG don't made them furthermore, it's obsolete.
 
I bought it almost good as new and checked by AKG.

I bought these on clearance back in 06. No regrets whatsoever. They are so comfortable, you'll swear they aren't even there. They sound great even with a relatively high impedance coming straight from an iPod. Most recently, I drive them with a custom kernel enabled Nexus S 4G. I have a FiiO e17 in the mail though, so I'll have to re-evaluate. 
 
They are also very visible and if someone sees you with those on your head, they wont come strike up a conversation. 
 
The only downside for me is leakage and how fragile they feel. I guess I put too much stress on the cord because one of the internal connectors actually broke off the driver on the right side. It was easy enough to re-solder, but still, thats a concern. The leakage is only a concern becuase I primarily use them at work and like to crank it up. 

Setup: EMU 0404 -> Corda Aria -> K501 (ver. 1.0)
 
This headphone is ideal for acoustic and classical music. I love the airy character of the sound that makes lively. Great for quality live records. Overall detail is very good especially on the midrange. The bass is punchy and full with a powerfull amp.

Had this for more than 15 years, got a pair of 702s but the 501s were still my preferred.
 
Just an awesome headphone

I heard this headphone a couple of years ago and fell in love with it. Since then I have been trying to find a mint pair at a reasonable price. This is a "poor man's electrostatic" as I have heard someone calling it. The mids are beautifully open and airy without a hint of hardness. The treble is soft and crisp. The bass is well defined but not in excess. They ar erelaxed, pure and genuine sounding. All in all, a legendary headphone. It's a shame AKG discontinued it.

This headphone is one of the best headphones for classical music. Others in the same price range would be Audio Technica ATH-AD900, Beyer DT880, AKG k701. I think that k501 can even challenge some other dynamic headphones in higher price ranges. It is definately better than Sennheisers offerings HD650/600, for classical. This is a real classic, one of a kind.

This is my first non-thread review. I gambled on the AKG K501 and fell in love with it. It is considered by many as one of the best headphones AKG ever produced, apart from the polarizing K1000. It is well known for a very natural and realistic tone or timbre which is something I'm very fussy about. IMO, the K501 nails timbre allowing music to sound almost as good as the real thing. The midrange is simply beautiful. It is slightly thick and lush while maintaining all detail. I wouldn't consider it bass light, my AD900 has less bass. I think the K501 is almost perfectly balanced.
 
However, it is not without it's own flaws. Compared to modern headphones like the AD900 and FA-003, it has less overall detail and extension on both ends. But it has the soul of a true classic headphone. One that focuses on producing beautiful mids instead of trying too hard to nail the frequency response. Just as another reviewer said before me, this is one headphones that every audiophile should have in their collection, especially at the price they can be had. I repeat, you probably won't get much better midrange than this unless you're willing to spend a lot more.

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!