I have AKG K712 Pro headphones which are very fast and responsive, but need a good deal of drive.  This is where Project Ember really sings.  I used the default impedance settings combined with a new old stock Raytheon 12AX7 organ tube.  This is an amazing combo as the detailed warmth on the Ratheon perfectly compliments the sometimes over analytical K712.  The result is an extremely detailed yet warm listening experience.