This is a review of the Crack amp driving Sennheiser HD800s. I haven't had the chance desire to audition it with any other cans. The price is for the amp ($213) and the Speedball upgrade ($110).   I've had the combo for about two years, and have several hundred hours on it. The Crack is an OTL headphone amp kit, designed for high-impedance phones. Assembly is easy, the directions are excellent, and support on the Bottlehead forum from Dan Schmalle and his very active, knowledgeable customer base is superb. After living with it for a while, I...