You are again, appearing to focus on some very specific user cases, just as your own!! Ideally, I would agree with you that we want high SINAD across the audible range. Since things are not ideal in the mid to upper mid price range gear, tradeoffs bound to happen. In that case, the SINAD vs measured output levels graph that the mid range gear such as Onkyo, Denon, Marantz, Anthem's AVRs/AVPs seem reasonably good.
Take a good look of the Onkyo RZ50 (I picked it because it is a current model, same with the Denon 3800):
Yes, SINADS is lower at the lower output level, in this case it might have been 500 mV ore lower, we don't know for sure but let's assume it is 500 mV, so it is about 88 dB Vs 96 dB at 2 V. That sounds like a lot, but consider the following:
1) 88 dB SINAD, or 0.004% THD+N is pretty good and I am 100% sure other reviewer such as Gene or JA would consider that level low enough to be considered below the threshold of audibility. Even Amir would probably agree though he might say that depends on if it is distortions dominated.
2) Even if -88 dB THD+N is audible to enough people in their HT rooms, because they have super quite rooms, unless they have speaker with very high sensitivity, how likely are going to be bothered by the difference between -88 dB and -96 dB when listening at spl well below reference leve?
Example: 88 dB/2.83V/m speakers will produce 78 dB at 10 ft, -88 dB THD+N is -10 dB below the fundamental, or -30 dB below a noise floor of 20 dB average and that's even if that -88 dB is all noise. How likely would most HT users be bothered by that level of noise related to the noise in a quiet room? On top of that, in real world use, there's also masking effects. Keep in mind many users find the likes of the NAD T758V3 or Anthem MRX that offers SINAD below 70 dB at 2 V, presumably much lower at below 500 mV.
That's the Onkyo, now back to the Denon 3800:
Here you see >90 dB at below 300 mV
So, even when you listen to classical music, or movies that has a lot of "quiet parts" that you refer to, it really won't be as serious as you seem to be suggesting.
To be clear, I agree with you and others who pointed out the fact that we do need SINAD to be high at the low output levels, but let's not help perpetuating the myth that it is a common issue, when it isn't. I am not sure if you are just trying to make a point by exaggerating the potential issue somewhat, but it may end up misleading others to think it is an issue for them when it may not be at all.
Again, I agree with you in principle, but it would be great if we all get less extreme in making our points on matters that are not always black and white.