I've just built the Crack (+ Speedball) for my HD600, and like it.
But, like others have mentioned, closed back headphones are sometimes a necessary. I already have the MrSpeakers Aeon, but looking for a tube amp or hybrid to drive them. These are 15 ohm resistive impedance (I believe the Ether are 23 ohm).
Any thoughts on driving this load with the S.E.X. 3.0? I'm assuming the 4 ohm tap would be used.