I bought the Rega DAC from the Audio-T sale without listening to it. I agree that, at first, the sound was indeed "shut in", with seemingly no dynamics and little in the way of treble. Then, a few hours later, it suddenly sounded amazingly dynamic with incredible interplay between instruments. This soon stopped as it began to break in further. Would this come back?
After being powered up continuously for 14 days, it was still sounding better than it had on the previous day. One thing that worried me initially was that it wasn't always easy to make sense of the music. For example, with Pink Floyd's Any Colour You Like, the drum beat didn't quite fit.
Any worries I had have now evaporated. The Rega DAC sounds superb. It does both the flat earth stuff (which is the more important thing in my opinion) and the round earth stuff (even through olive Naim amps). One thing I note is that it does sound better when using the digital SPDIF input than when using USB. This is partly a result of the computer being switched off (so it's not making a noise). However, I believe it sounds less "thick" and bass heavy when using an external player (rather than a PC). It did sound slightly brighter though. This may be the result of the digital cable I'm using (something that came with a computer graphics card and I don't know what type of cable it is).
In other words, don't begin to make any judgements until this DAC has had at least a fortnight or more in being powered up. All I can compare it with at present is my NA Hyperspace TT. I haven't made a direct comparison yet.
Andy