It really goes both ways. There are folks on both forums that are polite, rude, defensive, aggressive, trolling, sincere, prejudiced, etc. I haven't found an online forum yet that is free of at least a few abrasive members.
Sure that's true as far as it goes...people are people.
However, there is something inherently conflict-producing in the very approach or mentality of those who hold their subjective perception as superior to any objective evidence. It does not allow for finding out you are wrong, or any real way of settling issues. It's just one's personal experience, it's true, and that's that. Any questioning of this is viewed as a personal attack and is responded to with some form of ad hominem because of this. Because, since they have no other way to settle a disputed claim...what else do they have?
You can see exactly that dynamic playing out in the PS Audio thread. The "skeptics" are at least coming to the question with ways in which they can be convinced of the worth of the product, with some basic idea of what type of evidence would settle the question and that they would accept - e.g. some objective measurable evidence (and or listening tests controlling for bias). This is a way of saying
"I'm skeptical of the claims, but I'm happy to be shown I'm wrong, and here is how we can determine if you are right and I am wrong." I see civilized technical arguments and questions being offered by such skeptics on the PS thread. But since the purely "ears only" side seem to have no such evidence to present, and often won't even recognize the relevance of such evidence, they are driven to talking about the character of anyone who voices skepticism. Various versions of Ad Hominem.
The point being, it's not JUST about an individual's character that can put the death spiral in such discussions. It can be the sort of epistemic dogma that one has adopted (without realizing it's dogmatic undertones). And of course, a lot of this is based on ignorance of science/engineering/human psychology as well.