It may have been covered to some degree by your discussion in the
context of digital, but to me, i think it goes way beyond hiss- it is the
resolving power of the system to pluck out the nuances of the music or
voice against the background at low levels. The more 'absent' the
background, like ambient noise, the better you can hear all the little things
that are going on in the recording. (rough analogy would be trying to hear a
conversation in a noisy room, compared to quiet space). i think there is a
tendency to
turn up the volume to get more reality, and while that makes it 'louder,' it
doesn't make it more real sounding- in fact, sometimes the opposite
happens; the natural volume of the recorded performance is exceeded and
rather than sounding more lifelike, the system sounds like a sound
reproduction system. So, one 'test' (not really a test in the scientific sense,
but sort of a 'by thumb' way of getting the sense of a system) is how well it
resolves stuff at low volume. Not 'detail' per se, but the nuances, the
shading, the dynamics (differences between loud and soft) at a lower
overall volume level. if a system can do that well, i think it has a low 'noise
floor' in the sense that the music is presented with less ambient junk
around it and
stands out more clearly against the so-called 'black background.'
The point above about the 'natural volume' is a little different, and is just
that there seems to be a db level on playback where the recording just
sounds right, given how it was made, your system, room acoustics, the
type of music, etc. Kinda 'clicks in' just like getting the VTA on a tonearm
has a 'right spot' for a particular record.
And while the recording itself may have nothing to do with the noise floor of
the system, a system with a lower noise floor should be able to benefit
more from a good recording, if the above makes any sense.
my 2 centavos.