I wonder; since quite a silent room is ~30dB SPL, and considering maybe a 120 dB SPL transient, this is only 90dB. Is >100dB SNR a safety margin ?
30 dB is not silent room, that is pretty annoying noise level.
Powerful 12cm fan for PC has about 25 dB noise.
Human ear can listen sounds at about 0 dB.
Gun shot has about 150-170 dB.
So if you want audio system which can realistically reproduce gun shot sound and at the same time has no audible noise, it should have at least 150 dB dynamic range, which currently is not possible with current audio technology.
But the things is much more complicated, because human ear has non flat frequency response, while DAC specification shows SNR for a flat response. It requires about 40 dB more dynamic range for DAC in order to cover full dynamic range of human ear for all frequencies.
And there is needs to take into account that audio system has sound volume level regulator and user can change it depends on the situation and high gain should not lead to audible noise. It adds additional margin requirements for DAC SNR.
In reality there is current technology limitations which limits DAC SNR at about 120-130 dB. So, at this moment, there is no ideal DAC which can cover full human ear dynamic range with no distortions.
But the mains noise can be filtered to the level at which it cannot be audible. I'm sure that 50 Hz hum sound from transformers, wires, electrical panels, etc has a higher sound level than noise floor which a good sound card can provide.