short version: Amir insisted that he measure at 4vrms, if I understand correctly, which creates more distortion, however this unit would not be used like that by most (if any!) users, and at 1.6-2 vrms, its distortion ratings are pretty flippin stellar.
Amir's rating is based on his ego rather than common use. Pretty much what it boils down to.
I think if you take the time to research Amir;'s review discussion thread and the HTP-1 Users thread from around that time, you'll find all the info needed!
Keep in mind, he re-measured the Denon 3600? after denon called him on some foolery. He would not devisit the review of the HTP-1, though he may have made an addendum far down in the thread.
That's not quite fair, Amir often stressed the "audibility" part and that his "harsh" comments were based on SOTA 115 to 120 dB of SINAD, not that anything below even 75 dB would sound bad. As for the 4 Vrms used, that's pretty standard for balanced output, or 2 V for unbalanced output. Gene typically do the same, so are Hometheaterhifi.com and stereophile. In fact Gene has been emphasizing the need for clean output up to 2 V, or 4 V unbalanced.
He did not re-measure the 3600 either, I think you got that mixed up with the 4700, and he did admit his mistake on that one.
He had the Monolith ranked top for a while based on the reduced output to 2.7V (balanced/XLR) for a while, until it was passed by the Denon AVR-X8500H. For the 4 V output test, it still ranked quite high, only a few dB lower than the Denon and Emotiva's. For $4,000, I do think Monolith could have done better, and Amir gave it a "passing grade", that seems reasonable.
What is it about the HTP-1's measurements that he should revisit? If there are mistakes in his measurements, people should let him know, like Denon did on the AVR-X4700H. In that case, Amir should have been more careful and investigated his results before publishing, but in that case Denon did have that downmix issue that ultimately render Amir's test procedure problematic. If they did the test same way Amir did, they would have the same poorer results due to the downmix issue, that other products don't have. I don't listen to 7.1 downmixed to 2.1 but quite a few ASR members said they did, and some of them would not buy Denon because of that issue, so mistake it was, but the result turned out to be helpful regardless.
Monoprice HTP-1 Home Theater Processor Review | Audio Science Review (ASR) Forum
And yes, if you don't need more than 2.7 V (XLR), 1.35 V (RCA), that unit still has the second highest SINAD, only 1 dB below the Denon, that's within margin or error. That's just measurements. Threshold of audibility is a whole different story, it would be debatable anyone can tell the difference between even 66 dB SINAD and 95 dB SINAD. 66 dB SINAD is 0.05% THD+N, seems good enough for me.