So I've had the Beresford DAC connected up to my system for a few days now, and I am quite impressed. I haven't done blind tests, but with my favourite songs that I listen to often and know well, there is definitely a change for the better. I find that the different parts in a song are more defined, and the bass especially is improved.
I would be interested to hear what others have to say about this, as I know from reading the forums there are others thinking of adding a DAC to the airport express (JohnDuncan!).
Like I say, I have not done blind tests, but I'm happy with what I hear as an improvement, so I guess thats what matters!
Thanks again to those who answered my questions earlier in this thread!
Macbook - Airport Express - Beresford DAC - Rotel RA05 - B&W 685