-That different DAC's can and do sound different from one another is no longer subject to any legitimate debate.
-The degree to which DAC's sound differently from one another (as perceived by listeners) is subjective and entirely subject to debate.
-That people have differing levels of hearing ability is not subject to debate. Some people are smarter, some have better vision, some have a better sense of smell or taste, etc., etc.
-It is entirely possible that a particular user's system is not sufficiently resolving to reveal the sonic differences between DAC's, or may reduce the differences to such a level as to make the differences nearly indistinguishable. My guess is the strong majority of the people that frequent this site have sufficiently decent gear that sonic differences in DAC's should be apparent (to varying degrees).
-It is possible that some people don't really know what to listen for, or don't have the appropriate attention span, or are using demo music that does not illuminate the differences.
So, IMHO, if someone does not hear any difference (or very, very little difference) between say, the Bimby and an ODAC, then some of the possible reasons are:
1. The listener does not have sufficient hearing ability (sorry, that is life, but I am sure you have other excellent qualities
). Back-handed 'golden ear' compliments from those that can't hear differences won't change this fact.
OR
2. The listener's gear is to blame (unlikely for most on this forum).
OR
3. Increase your focus/attention, choose different music. If that does not work, see points 1 and 2 above.