I have two audio-gd's DAC, one has ACSS out (REF7HE) and the other has diamond differential output (1541S2). They sound different but either of them are excellent DAC.
Superjohnny, I could imagine your REF8 is an excellent sounding DAC also, it takes times to burn in to do its best, be patient.
For amp to use your headphone, why not Phoenix if budget allow?