I can't answer most of this, but I can make some huge over generalizations about tubes in DACs. And, that is, while I like tubes, and consider them to generally be the best active way to produce voltage gain, there isn't much place for them in most DACs outside of marketing, at least those using current out chips. The reason is that a current out DAC chip works best into a low impedance. That means either an opamp or the source/emitter/cathode of an active device. The input impedance for one of these devices is ~1/Gm. With a BJT or a FET, this is easily under an ohm. However, even a super duper high Gm tube, like a 7788, is still orders of magnitude higher. And, those are rare and unlikely to be seen in a commercial device. The highest Gm tube you are likely to see in commercial gear is a 6922, and it has an input Z around 80 ohms which is too high. Moreover, most higher end DAC chips run a pretty high current out, so you'd need to really push a tube like the 6922 to carry that sort of current.
Secondly, there is also often a buffer stage, but again, sand devices just work better here. A cathode follower really offers no benefit over a source/emitter follower sound wise and has worse drive ability. And, it costs a lot more to make. And it is likely noisier.