... that persisted in blind testing, which isn't usually a thing baseless predispositions do.
I did. They say, "I wanted to get this review out quickly so did not spend extensive amount of time listening.... I tried with a few tracks to tell the two amps apart but I could not. In more careful ABX scenario where I can focus on specific segments I might do better."
That doesn't seem at all incompatible with someone finding differences when spending many hours over multiple days doing careful comparisons. If you don't hold a strong preconception that it's impossible to hear the difference between amps, the evidence here seems to point toward it being possible.
While I agree with you more than the bossman in this instance. The problem is, the testing methodology is unpublished. We cannot replicate his experiment, so it's tough to grant full backing. But in that same fashion, the bossman's is similarly, if not more non-descript about the testing (but based on how he does tests with other things in the past, I can be sure his tests are rigorous, but in this case we have no idea what he actually did either).
Like if I said "yeah man I can blind test THX 789, 877, Archel Pro 2, Atom, Element 2, Heresy" and "I level matched them all within 0.1dB of one another, and I could tell the difference between all of them using FLAC files on my computer". You wouldn't take that to the bank I hope. Even if I had a review site to publish my review similarly.
When one person is doing a whole review with no one to at least attest to any of this actually happening. Most of this is taken as truth of non-truth with some level of faith. But if for instance, the person doing the review regularly participate in ABX tests with other people helping him, and has a history and publishes is work for proper archival purposes of re-running the experiment if others want to try themselves.. Then a review he conducts one time without a person, you can probably be sure he's most likely not lying or conducting the experiment in some shady or inadequate manner.