How important is S/N Ratio....?

If you listen to loud-all-the-time rock music S/N is of no concern. For quiet classical music 90 dB is OK. 100 dB or more is for neurotic audiophiles who put their ear to the speaker when no music is playing. (By the way, most of my stuff is spec'd at 100 dB or more. It's no big deal).

Be careful as there is no defined way to measure this - it is the noise with respect to a "reference level". If you stick to pro audio gear than the S/N ratio on balanced outputs is usually referenced to 1.23 Volts.

If you use consumer RCA then S/N is often referenced to 0.316 volts or whatever the manufacturer "decides" (4 or more times less stringent)!

So the S/N specification on pro gear is significantly better than the same spec on consumer gear and on consumer gear it is misleading to compare specifications anyway!

Oh and by the way - the reason gear may sound different is that you can actually hear about 20 - to 30 db BELOW a noise floor (yes our hearing is quite good). So what matters more is the type of noise...if it is just thermal background white noise that is totally unrelated to the audio signal then you have a great piece of gear. If you have equivalently low levels of noise but when music is playing you have within it some correlated noise that follows with the music (harmonically or IMD related) then this gear will sound much worse (even if they both share the same "on paper" S/N ratio)

This is why gear sounds different and some gear has a blacker background where subtleties stand out a wee bit more. Remember our ears are like harmonic analyzers - they pick up vibrations that are cyclical in nature... so white noise is much more benign than correlated noise which has a specific frequency content and which changes in response to the music signal.

if it is just thermal background white noise that is totally unrelated to the audio signal then you have a great piece of gear

I thought I would add that this is why analog tape hiss or Vinyl road noise (apart from wow and flutter) is not a big deal - we hear the music through this provided it is random noise.

" If you have equivalently low levels of noise but when music is playing you have within it some correlated noise that follows with the music (harmonically or IMD related) then this gear will sound much worse (even if they both share the same "on paper" S/N ratio)"

So what your saying is distortion effects the S/N or not...when measured.

"if it is just thermal background white noise that is totally unrelated to the audio signal then you have a great piece of gear"

Ok so more inner detail...subtleties....Clarity......Focus

But let's ponder this.....if distortion effects what we hear......wouldn't the S/N....true measurement...measure the distortion present in the amplifier. So the higher the S/N the lower level of distortion present in the device..amplifier.

That would in theory effect....soundstage,imaging percieved and also bandwith of the amplifier.

Also the componet with the poorest S/N would be the weakest link in the chain...hmmmm.