Dynamic Range: As was well explained by Mr. Julian Hirsch (Stereo Review) in the 70's, it is the range between the loudest sound and the lowest sound. The loudest, might be the loudest we can play, before getting into large distortion figures. Regardless of analog recording, nor SPL.
The lowest, as defined here, is the noise floor. With electronics, it can be relatively low, as Johnson noise, Op Amp's noise etc' with analog sources (vinyl or tape) is the rumble or hiss (-50dB) or CD with S/N of -100dB or better.
Mostly, such dynamic ranges, also need to consider the lowest sound (music) ref. to the noise floor. A 6dB ration, is as good as a tranche line phone of WW-II. So that would be nice to have a decent distance between the two. With digital sources, as CD (or better), the lowest bit (LSB, to none) is
at -96dB. at the lowest effective bits are about 4-6 bits + dither. So S/N below -115 dB is fine.
Another issue, is the fact that we use a serial system: Source, Pre, Power etc'. For such a set, the combined S/N would be the worst out of the list.
If it would be a FR figure it would be a RMS of all FR.
As wrote B4, the environmental noise is way higher than the digital sources or fine SS amplification.