5 days in and not a single reply from the OP.
The Audioquest Nighthawk are excellent and will work with any headamp.
Many thanks for all the advice chaps. Very useful indeed.
The HD600s seem to be very popular and nobody seems to be advocating wireless. Is this purposeful?
I will use them with the Naim DAC V1 as I have no desire to change this and its headphone output seems extremely capable. Suggestions for other headphone amps are thus a bit premature at this stage.
I am happy to go to the top of budget for something special on the s/h market.
Hmm... bearing in mind the rest of your system you might find HD600s a bit 'shut in' and 'cuddly' in the bass.