Mojo ... give the guy some numbers ... What do you think should be the cut off for those ... where it become just a spec and not audible...
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.