For S/N, you have to know two things: ambient noise of your space and the level where your ears compress. Ears compress at 120dB. Ambient noise is probably no better than 30dB. This means a S/N greater than 90dB is practically of no importance.
For THD+N, it's generally accepted that anything less than 1% over the entire audible band and volume levels is inaudible.
The figures above are for the entire *system* and not individual components.
ear compression ... interesting way to come up with a number ...
1% THD was the number I was using back in the day (because that, as you point out, was considered inaudible). I*d like to see something below 0.5 for my set up just because. Any thing below 0.1 I would call just a number. Using the same 10x cushion for your S/N number puts Noise at 100dB down ... I would not pick one system over another to do better.
BTW I doubt distortion is a real concern in any reasonable pre amp stage today.