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).

There is no "standard" dB level for a jet plane. Some business jets are so quiet that we don't know they have arrived at our little local airport until they taxi up to the terminal. On the other hand, I have had occasion to work with an F15 fighter in a flight testing project. Let me tell you, a full afterburner takeoff from a couple of hundred feet beside the runway is an awesome experience. We were required to wear ear plugs, but even so it was loud. I believe that even one unprotected event would do damage.