I'm writing just to add my voice to the chorus: The Crack headphone amp is an excellent kit. It was my first build ever and I'm still amazed that the thing actually (1) works and (2) sounds great. The supplied RCA 6AS7 and Toshiba 5963 tubes sounded pretty good with Etymotic ER4-S IEMs, but I replaced them with a Tung-Sol 5998 and an RCA 6680 "clear top." The result: lower noise floor and the amp really sings now with the Etys, and with AKG K701s. Many thanks to everyone at Bottlehead for a great experience. Crack was my first kit, but it won't be my last.
Paul