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AKG K518LE is one of my favorite headphones!
I tried my (rebuilt) K518 on today after litsening to my new K550 since christmas eve. I thought the K550 would blow them out of the water and make them sound useless, but I was amazed how good they still sound! Sure, the soundstage is just a little bit wider than my head and they are not as detailed as the K550, but the bass is addictive and compared to most other headphones I've tried they are still great. The only problem was the pressure on the ears, as usual...
What have you compared the K518 with? It would be cool to try them out side by side to a pair of more expensive Beats and see what comes out on top. :)