In the first link, down at the bottom of the page is a graph. It's labeled:
"THD+N vs Power - Relative to 1 W"
I wonder what the "Relative to 1 W" part means?
I also don't recall ever seeing a graph where the noise doesn't increase as the power decreases.
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Here is a very nice DIY chip amp kit graph.
You might be overthinking this. Don't take offense, as I did this for me as much as for an answer to your question as it is a good point that I had just glossed over since I intuitively knew what they were on about.
DBR is db relative to something, db are just ratios, so they need something to "be attached to". In their graph, the y axis is relative to 1W of power (note their little circles on their colored lines relative to 1watt on the x axis)measured probably on a resistor. I don't like math, but db are just an easier way to represent really big or small numbers really.
thus:
Odb we will reference to 1W thus dbr , dbrelative is to our defined 1 watt
+100 dbr (watt) is then 10,000,000,000 more of watts
-100 dbr (watt) is then 10,000,000,000 less watts
So, simply they are showing that their amp has less THD+N than the other guys using their selected thing "to be attached to", ie 1watt across some load as their 1 unit. Their dbr, dbrelative to 1 watt might (agree with Don here as well) also have used an "A" weighting filter so it a little bit better corresponds to how our ear roughly evaluates loudness (loudness is our internal amplitude reference from not hearing anything hardly to a bomb going off next to us)
Clearly, Benchmark need to better explain their stuff, use more pictures etc, so folks can really grasp what they are saying, they should say their THD+N is 150 thousand times lower or whatever it is than their competitor, that is easier to understand and sounds really impressive...