SB Duet Sound Quality Improvement?

Recently set up the duet. Originally went rca and found the s/q acceptable but not close to my cd player. Switched to digital optical and the sound opened up. Image front to back and side to side became stable and well spaced, bass had the presence that I am accustomed to and the mids sounded very good. Only downside noticed so far are the highs. There is an etched grating quality that I want to tame and I feel that there is more to be had. Many seem to think the squeezebox is capable of replacing high end cd players given the right approach.
Current system:
sb receiver to Lite Dac Ah to B&K Reference 20 to Bryston 3bst to Gallo 3.1.
I'm thinking about a tubed dac or linear power supply to tame the highs and possibly round out the other areas. Which would be the best starting point?
Have just begun looking at tubed dac's and the paradisea seems to get very good reviews but I am open to suggestions. Price range will be less than $700.00 and used is fine.
Thanks
Steve