I've heard a buddies pair at his place several times. They are very dynamic and detailed, very open for headphones. If you have a big noggin they can squeeze the bejesus out of you. I didn't want to stretch my friends band, I'm sure it could be improved. As Swampwalker mentioned, the AKG's offer zero isolation. They stick out from your ears a bit, and can be heard by others in the room. You'll need to drive them from speaker taps. I've tried them with a Sonic Impact which paired quite well. While I enjoy these, I can't say I prefer them to the 650's driven by the same amp.
