I thought that's what the definition of SNR was? I could be wrong.
Just to be clear, I have no doubt you know what SNR is. I was only puzzled by your remark as quoted below, and only the highlighted part:
"If the content is 100db louder than the noise, such as it is in all those components, then I doubt it would be audible."
Am I right to assume you were talking about the recently linked reviews of the NAD T758 V3, Anthem MRX 520, or the Yamaha RX-A1080? If it was something else then please ignore my questions.
If you were in fact referring to the recently tested AVRs over there, none of them came close to -100 dB if referenced to 1 W output. To me, SNR at rated power, or pre out output are not very helpful as most people don't push their amps anywhere near their output limits. My amps typically would average below 0.2 to 0.4 W per channel when I listen to music, slightly higher for movies, but obviously ommv..
Below are the SNR (for simplicity I rounded them to 1 decimal place and use the worse channel) measured by Audiosciencereviews.com on the units below:
Analog in
T758 V3 - 89.5 dB at rated output
RX-A1080 - 101.3 dB at 5 W output
MRX 520 - SNR not measured, but DR at rated should be the numerically the same and it was 99.98 dB, but was at rated pre out of 2V, that's above rated power output.
So at 1 W, the the NAD would be in the neighborhood of -70 dB and that would be well below CD quality (16 bit, or 96 dB).
The Yamaha did a lot better, but at 0.5 W output, SNR would be down to about 88 dB, still well below CD's.
Of the 3 that site recently measured, the Anthem had the best SNR but as noted, he measured it at 2V output, that means if referred back to the same level as the others, it would fall short of CD quality too.
It is very difficult to use their measurement to compare gear because his reference points are not always consistently the same. I (probably others too) have given him some suggestions, now that he started to measure more AVR types of gear.
I do agree "all" of those tested would not be a concern for a lot of people, in terms of audibility of the noise related issues. I don't think it is because of the SNR of their gear being good or bad enough on paper, but most likely because a lot of people don't have very quiet rooms to begin with but now this is just my guess.