Thanks much guys!
But I must confess, DACs are much more complicated than I thought, especially things like I2S...
Anyway, some perspective, I currently use the DAC in an Audio-gd Fun, with a powered Audio-gd Digital Interface, feeding a Glenn OTL. My headphones are HD700 slightly modded to tame the 5-6k treble spike. I am primarily a tube-rolling guy, but colleagues have suggested that I might be surprised at how much better a newer, dedicated DAC might sound. And from the comments above, I am inclined to think that either the Master 7 or the R2R 1 would be a major upgrade. By the say, would the Digital Interface still be worth using with either of these?
Thanks again.