DAC's only:
Audio GD NFB 3.1
Cambridge DacMagic (also has balanced outs, if you ever decide to go that route).
Music Hall DAC 25.3
Arcam rDac Solo
Musical Fidelity V-DAC Mk II
Lower price options for USB only:
HRT Music Streamer II
DACPort LX
DAC + Headamp options:
Yulong D100
Matrix Mini-i
Audio GD NFB-5
Audio GD NFB-10SE (balanced DAC and balanced headamp) - would be a great high power SS counterpart to the Lyr.
For under $600, If you're perfectly happy with the amp options you have, something like the MusicHall 25.3 or DacMagic are the way I would go. If adding a balanced SS amp into the mix is attractive as a bonus to a very capable DAC, I'd get the Audio GD-10SE