... internal processing...
By internal processing I do not mean that familiarity leads to recognition - although there is an element of "ear training" that plays a role in how we all hear. The question is not familliarity as much as how much internal processing you are actually doing to have your mind say "ahhh I know what that is/means."
An example of this *sort* of thing is when you randomly tune your car radio to the middle of a song, one that you hear all the time, and it takes a few seconds to figure out which song it is, as you have tuned into a spot that is perhaps in the middle of a musical phrase, so you don't have the benefit of a "start" point (musically or lyrically) to guide you.
What I am speaking of is similar, but more subtle in that the clues that tell you where/what on a hi-fi system, and really give you the detailed tonal and spatial information are much more minute.
Perhaps one of the reasons that you can "hear" more on your own system is that you have given your brain an "algorithm" with which to quickly process the raw sound into intellegable information.
So, the test then is to be able to walk in "cold" (no pre-programming of your brain, just "natural" sound algorithms) and be able to *instantly* recognize all the sonic clues! Of course, this is not a constant, nor is it (as far as I know) measurable (yet), but if you think about what happens when you are out in the world everyday, or at a non amplified live performance, you don't have to strain or work at all, it all just *IS* <-- kind of a zen thing.
Hope that explains...
_-_-bear