I'm debating what DAC to get to pair with the Nagra HD Pre. Currently I just use the DAC that is internal to my Esoteric K-03XD SACD/CD transport. I understand it to be extremely comparable to the stand alone Esoteric DACs, if not identical. It's excellent, but there is room for more. Although I tend to think the DAC is the least benefit for the cost, given technology has progressed so fast. Cheap DACs can be pretty amazing, so it has to be a serious step up.
Contenders are one of the MSB line (probably the Reference; the Select just seems overkill, but maybe not); the Nagra HD DAC X (which I note you have); and the Mola Mola T- (fish name - Tiburon? Talapia? Something exceptionally stupid); and the top of the line Weiss model.
I've literally never heard any of these except the MSB Select, and it was at a friend's party, so I really didn't have a personal opinion about any of them. Based on reviews, the Mola Mola or the MSB seems to be the betters choice (ironically) for the Nagra HD, as both tend toward "bright" and "sharp" if there are any complaints and I think the Nagra would resolved that -- whereas I hypothosize (without knowledge) that two Nagras in a chain might be a tad sloppy. Again, that's my theory based on zero actual knowledge.