Michael,
I've heard the Duet once, at a freind's Brooklyn, multistory, high density condo. It was on a Creek CD player and integrated amp. In that specific context, the change wrought with the Duet (as opposed to directly into the mains outlets), was a substantial change for the better. IMO, it was a very cost effective upgrade for HIS system. Topround (Mike) also purchased a Duet, and has reported a substantial improvement in his digital components, but also feels that it constrained the dynamics of both his Mapletree tube preamp, as well as his VTL85 tube poweramp.
Given the current sale price, and if you can purchase it with a 30day return priveledge, I'd suggest that you evaluate it in your system.
FWIW,
Paul