Any proper businessman with common sense would respond professionally regardless of the treatment he is getting. I have been on the other side of this fence many times with my own products getting reviewed. You always act proper, kind and helpful. That way, you at least look good in front of your potential customers even if you can't turn around the reviewer.
Here, John's job was impossible anyway. The measurements are ruthlessly revealing of a poorly performing DAC. Construction of not one, but two showed the device to be unsafe, sold without legal certification putting aside all the dubious "audiophile ideas" stuffed in there with no verification.
Being an interactive forum, people are naturally outraged and indicated so. The treatment John is getting is factual based on his debating tactics, not answering simple questions, showing clear lack of understanding of electronics he is playing with etc. He needs to feel the pain as to do better, much better. Us rolling out a red carpet for him would cause him to continue doing the disservice to the audiophile community.
John's rude remarks have also broken our rules here for proper conduct. I could have banned him multiple times for that but have restrained.
Finally, remember, I am an advocate for consumers, not manufacturers. You want to do the latter, be my guest but don't ask me. The tested products here are so bad that no amount of outrage is enough to describe it. If there was a time to make the comment you are making, this is NOT it.
There's nothing wrong with being an advocate for consumers and not manufacturers. I think the practise of some of these magazines refusing to say anything negative about a peice of gear reviewed is horrendous and so ASR is a breath of fresh air. BUT.... there's definitely a way to go about it though. Your post boils down once again to "if he deserves it, so be it" to justify poor behavior.
You've said before that what matters is the measurements and the facts. It looks like there is a lot of feelings getting in the way whether it be a sense of "outrage" or the history you've had with him which - appears to have been pretty irritating. However, the facts will bear out in your favor everytime. Allowing a manufacturer to be a scratching post won't.
Let's examine. If you had talked with the manufacturer before (and bear with me it's possible that the manufacturer mayhave posted fake info)
You may have found
That it could be run on a battery ("supposedly" providing better reuslts).
The correct model.
The correct manual for the model. (The user may have printed out a different manual as he himself was not aware of what model it was.)
The power supply may have caused an issue - and you'd have found out that the manufacturer does not provide a power supply. And then you could point out that the manufacturer refuses to supply appropriate details on a "adequately clean" power supply.
Some feedback as to his layout and refusal to use a PCB.
And more importantly - a working unit - which you could compare to the broken one giving more data.
It would have taken a five minute email - maybe. Then when you post the negative review, you'd have more credibility.
Right now you have the manufacturer pointing things like you don't know the model or how to measure it and refusing to cooperate with you. Is it a fair accusation? I don't know but you have given him the ability to say these things.
If you had contacted him and then posted the negative review with points refuting his assertions, he would have less to complain about and he wouldn't have a leg to stand on when not wanting to cooperate. A quick email exchange - take five minutes.