Signal to noise ratio!

Feel free to comment, I like to know what others think
Every time you see specifications (eg comparing CD with LP) you see a signal to noise ratio 102db or whatever. These "measured" signal to noise ratios compare the maximum volume of music with the hiss and humm in the component. Is this correct? We all know that some very silent components reveal a lot less detail in music compared with a good tube amplifier with a relative bad measured signal to noise ratio.
My guess is that there is a lot of noise in all components that is not present on it self, but take out parts of the music without being there if no music played.
Maybe there are some technicians out there who think the same and found a way to quantify this.

 

Controlled studies strongly suggest that the low level hiss plays a role in sonic perception. The absence of such hiss is sometimes perceived as an absence of air, high end detail and sonic stage, a dry sonic experience. This suggests that the subject is complex and hard to get one's hand around. It does not suggest that this is the only factor involved in reported perceptions concerning vinyl v. digital or solid state v tube (some sold state hardware can suffer from hiss as well.