I've gone over to SET amps, so what I look for isn't so much technical as musical. A liquid midrange without being syrupy, a smooth, sweet treble, and adequate bass extension are fundamentally important. Palpable imaging goes without saying, along with a sense of realism and naturalness. On the technical side low noise is a must, given that the speakers will be quite efficient, and adequate input sensitivity to mate with my favoured transformer passive line stage.
