These are my findings, after having modded about 20 different DAC's for 10 years, attending shows for 15 years and now selling my own DAC for a few years:
Above about $10K, most of them sound very good. There are some giant-killers out there, even at this price range though. Going to $20K or $50K will not necessarily get you better SQ IMO. More features like MQA, DSD, network access, and prettier chassis/displays, maybe. Features and aesthetics are nice, but I prefer the SQ to lots of features. Just my take.
Achieving great SQ is mostly a matter of implementation in the DAC, not so much the parts selection. PC board design, power supply design, power delivery design (decoupling, wiring, regulation etc,..) and the simplicity of the signal path are all critical. Choosing op-amps over discrete transistor design can be beneficial, however there are some really good op-amps out there that sound amazing as long as you don't use too many and feed them properly. I do stay away from really old D/A chips however because they tend not to reproduce the HF transients very accurately. One critical thing to consider when buying an expensive DAC is how are they doing the digital filtering. Aside from jitter, digital filters are the bain of good SQ.
Steve N.
Empirical Audio
Indeed, that is why IMO it lis best to do away with them altogether (the filter). Not sure I like pop amps at all. Much prefer I/V conversion with a transformer.