The question is not at what point noise become a problem. The question is at what point the SINAD and SNR indicate that there is a problem that needs to be addressed. No
OK, at the computer now so can give you a longer answer.
SNR and SINAD are engineering measurements that are automated and can be instantly made. To this end, when a company doesn't bother to make such, or produces equipment with poor ratings in them, they deserve criticism unless they answer question: what good did the extra noise and distortion do for the listener? In this forum, the general answer is that it hasn't done any good.
Both noise and SINAD can be bookmarked at either end easily. Get noise and distortion below -115 dB and you are guaranteed for it be inaudible for all people, all situations, all content, etc. At the other extreme, drag them down to say -20 dB, and everyone can hear the impairment.
What exists in between is a giant gray area. Assessing audibility requires either controlled listening tests, or psychoacoustic analysis. The latter is what I use often in commenting on whether impairments I find are audible. A jitter component at -130 dB is declared as inaudible. A jitter component -90 dB -- above threshold of hearing -- is also considered inaudible if it is close to our main tone (masking). So mere violation of above range doesn't make them audible. Spectrum matters and such is shown in my measurements (e.g. FFT).
You could complain that life is unfair if you can't just glance at the SNR and SINAD values and determine if there is an audibility problem. My answer is sure, it is unfair. In an ideal world, a Doctor's statoscope would instantly tell you that if you -- heaven forbid -- have lung cancer. But it doesn't yet your doctor always uses it as a first level diagnostic. He uses that plus a world of experience to know if there is cause for concern or not.
I asked my doctor once why he couldn't give me definitive answer on a question I had. He said, "you engineers are used to black and white answers. Our world is nothing but gray. We get some data and make educated guesses but at the end, it is not definitive." Our dog had a cist removed and they prescribed antibiotics. They said he doesn't need it but dogs being dogs, they may get the wound exposed to the outdoors and get an infection. So some prophylactic was in order.
Same here. I can't in the couple of hours I have conduct full controlled listening tests, or psychoacoustic analysis. I measure common metrics that tell me, as an experienced reviewer and engineer, whether performance was left on the table that could become audible. I express that opinion in the review. That word is key: I write both measurements and reviews. What makes the post a review is my experience and overall assessment of what I just observed. You could ignore that if you have higher level of knowledge and insight than I do. Otherwise, I suggest not second guessing that any more than you do that with your doctor when you have no medical knowledge or advice of another doctor.
In this case there is special dynamics: CHORD as a brand is known for over-engineering and impeccable attention to smallest impairments. Dave actually goes tens of dBs beyond threshold of hearing because well, he thinks there is audible problem there (without any verification per above). This halo reputation is going to lead people to buy this phono stage thinking they are getting the same, over-engineered products. Well, they are getting the exact opposite. What they receive is one of the worst measuring and engineered phono stages I have seen or measured.
Because of the above, Chord is better off going back and cleaning up the design. It will then better match their brand and reputation they have created.
Remember also that other choices of products exist without these problems at much lower cost. To that end, as I tried to explain with that washing hand analogy, you can buy them and sleep easy that you are not injecting a ton of distortion and noise that might be audible into your system. That is the value we bring here. To the extent you want to be dismissive of this service, then we have nothing else for you.