[SilverEars]

Are there any specs or measurements of a source that can tell you if the source will be very transparent or detail and neutral and outputting close to exactly how the recording was intended?

 

[kraken2109]

A flat frequency response with low distortion.

 

[bigshot]

Here are specs for the thresholds of human perception.
http://www.head-fi.org/t/645851/the-most-important-spec-sheet-the-human-ear
 
In general, 20-20 +/-3dB, distortion below 1% and dynamic range above 70 is transparent.
 
But prepare for some surprises, because there are an awful lot of people on Head-Fi claiming to hear things that require super human ears to hear.