HD650, and I already own the Lyr, which IMO has very, very good synergy with the Senns, particularly with some good NOS tubes. However, I've read so many good things about the Crack + Speedball with this headphone that I'm considering jumping the Schiit ship. The purpose of my post was to elicit opinions from people who have heard both amps. Oh, and I'd be buying a pre-built Crack, not doing it myself.