D600 and C2000
We are TAD dealer, so this is obviously biased, but we are big fans of the TAD electronics. The components are clearly not cheap, but if you open up any of them and have a look around, you can easily see where the money is going. Many of the internal parts are proprietary. Sonically, the results speak for themselves. The C2000 seems to be the perfect match for the M2500/4300 amplifier. The two balance each other out nicely. In isolation, I personally find the M2500/4300 to be good, but some people with smaller and/or reflective rooms find it to be a little bright. Adding the C2000 mellows it out in a really nice way. Together, they give a nice level of resolution and smoothness.
The D600, on the other hand, sounds amazing with and without the complementary C600 preamp and/or M600 mono blocks. The D600 sounds neutral and is very resolving. Some detractors argue that the D600 resamples DSD to PCM, so it is therefore not purist for the DSD crew. The design team felt it sounded better this way, though, which is why they chose to do things this way.
TAD electronics are extremely well constructed and deliver the goods after the break-in period (about 200-300 hours in my experience). I strongly recommend trying them out before drawing any conclusions, and ideally in the right combinations. We have most of the items in stock, so if anyone is interested, please contact me (sales AT ciamara DOT com). We'll help you make an informed decision.