I guess I'll answer my own question since Ian at Grant Fidelity has already emailed back...
Here is what he said: "600 ohms is fine, the PH-6 does 30-600 ohms, you would select the 300 ohm switch for headphones from about 150ohms to 600 is how it works. Of course, for headphones between say 100 and 250 ohms you could try both settings and see which worked best. This is an excellent sounding headphone amp. I run the TubeDAC-11 DirectOut into it and it is quite something!
Cheers,
Ian"
I know Yaqin is one of the better regarded Chinese audio companies, so this is a tempting buy for a first headphone amp.