When there are differences between the sound of DACS, it could only be in the analogue section. I have, for instance, a CD player with a tube analogue section, and it certainly does sound colored. That is what to look for if one can actualy, for the sake of argument, hear differences between DACS. There should be a lot that sound the same and a few that don't.
The ones that sound the same are OK. The others are not transparent; they are colored. That is something a DAC should never be, so avoid them. These would be either dreadfully designed cheap models or high end models that suffer from attempts to improve - or over improve to be precise - the basic, already perfect DAC. That is, perfect in that its distortions are inaudible, easy to achieve. It brings to mind the individuals (and companies) who look at a good design and decide the op amps are not good enough in a piece of gear. They substitute better thought of op amps and mess everything up, yielding a worse sounding unit.
In their arrogance, they forget that the designers of integrated circuits know better than anyone else how to use them. Op amps in particular are very application specific, even more than most ICs. Audio history is full of very over priced electronics that are great sounding designs right out of the manufacture's application books. I guess the fancy casework was worth the extra zero or two on the price tag. But not for me. I'm buying sound, not furniture, but that is another topic in sound science.
Another thing about op amps. Other than varying in noise level - from inaudible to even less I might add - they tend to be nearly perfect. This is partially because of ideally integrated layout and partially because they are so compact. It is highly unlikely that their audio performance will ever be equalled by discrete, non chip op amps. It is an uninformed prejudice that ICs can't sound as good or better than discrete component circuits. A visually clued observation that the little chip could not possibly be as good as all those big, expensive parts. A dual jfet input op amp is thing of wonder; do not underestimate it.
Bear in mind that I am not immune from liking my particular taste in distortion. I use a FirstWatt F2Jfet that is single ended, single stage, feedback free and class A. Nelson Pass, the designer, gladly reveals where and how much it strays from distortion free. But both he and I love the way it plays music. A similar situation, some tube amp users love the sound of their transformers. The point is, none of this thinking belongs in a DAC. If it is not transparent the benchmark for your system is lost. Get everything to be transparent and flat, then alter to taste. To not do so will indeed result in a never ending quest all right. Never knowing where you are or exactly where you have been.
Edited by Clarkmc2 - 7/6/12 at 11:59am