Wow, 600 ohms input -- what were they thinking?? Sometimes it's hard to tell whether such a wacky spec is real, or a result of "lost in translation". Based on your experiences, sounds like this one is real.
Preamps that will handle 600 ohms are very rare. I have no experience with Atmasphere's products, but from all indications I've seen, they'd be an excellent choice. Another route (as you learned with your Lyr) is to look for headphone amps with preamp functionality. I'd look for something tubed that doesn't use electrolytic output caps -- which are the typical way that cheap tube headphone amps handle the low impedance loads. I used to have original Eddie Current Zana Deux; it served very nicely as a preamp and had an output impedance of only 16 ohms (no remote, though).
Preamps that will handle 600 ohms are very rare. I have no experience with Atmasphere's products, but from all indications I've seen, they'd be an excellent choice. Another route (as you learned with your Lyr) is to look for headphone amps with preamp functionality. I'd look for something tubed that doesn't use electrolytic output caps -- which are the typical way that cheap tube headphone amps handle the low impedance loads. I used to have original Eddie Current Zana Deux; it served very nicely as a preamp and had an output impedance of only 16 ohms (no remote, though).