DAC differences are always a lot subtler than speakers or rooms and a lot depends on the quality of the listener (untrained vs trained listener, older vs younger ears, etc). In many ways, the listener/test subject can be a high quality test subject or a low quality test subject.
Higher quality power conditioners can even out differences between dacs at different price points, especially if they share the same topology and are made by the same manufacturer.
Swapping between manufacturers, topologies (DS Vs R2R), the chips in use, opamps vs discrete output stage, etc can let a test subject hear differences better.
Dac differences can become easier to hear if the test subject uses a BACCH processor... very subtle differences in the soundstage will become easier to hear with BACCH.
If the test subject processed his files with Daniel Hertz Master Class (Cwave), he may not hear many differences on some dacs at all...
etc
If it is very hard to hear differences in dacs consistently for a higher quality listener, there would be other heavier bottlenecks in the setup..untreated/incorrectly treated room, pooer setup, screwy speakers, amps, etc...