I discovered how nice it is to be able to roll the 12AU7 tube in the Hifiman EF5.
I discovered the Radiotechnique 12AU7, available from Upscale Audio. Gorgeous tube, with great extension on top and impressive bass. The perfect compliment to my LCD2 headphones.. I've been wanting more extension because the LCD2 is a little rolled off.
My EF5 is modded (by a guy I hired, Igor Kuznetsoff in New Jersey). He discovered there are TWO opamps to roll, both the visible one on top of the circuit board, and an invisible one beneath. He replaced both with a Linear Tech type with a very fast slew rate, but I don't know the model number (I can't find my flashlight and glasses to peer into the amp at this moment).
I've also used the German-made RFT 12AU7 and that is a bright and lean tube which is a nice compliment for the LCD2 if you feel the LCD2 is a little too rolled-off and fat.
I like the Radiotechnique better, though -- it's less bright but more extended on top.