To play devils advocate here - how do we know that Abyss did not develop that special WA33 in collaboration to make their headphones sound the best? We are all assuming here that they must be promoting it because they have a business relationship with them (same with XI Audio). But is it possible that they established those relationships because they believed those amps sound the best? Kinda "chicken-egg" problem, here.
Do they have a power to affect viewers' minds on what amp to buy? Yes, of course. They are the manufacturer of said headphone, hence (some) viewers will take their opinion on what to buy, or where to look first. This being said, from talking to other head-fiers, I get the feeling that everyone here is an intelligent individual, who can make their own mind about a product they want to try, no?
I never understood why do people get so upset about Abyss making an amp comparison video. It is in the end their opinion, isn't it? Or, are they breaking some consumer protection laws I am not aware of? I for one try to establish a baseline for a reviewer based on their opinions of gear I know well, and then based on that I try to take their advice on stuff I haven't heard. There is no substitute for trying yourself, demoing units and trying them for a week or two at least. With this level of gear, no one should be buying blindly. I know a lot of this stuff is built to order, so some of you are actually buying blindly, but I hope you have enough cash to sell and survive the re-sale blow, and call it a "journey" or an experiment.
For example, there is no single Youtube reviewer, nor a head-fier I agree on all points. Everyone has different ears, expectations, and likes different music. I think 1266 has issues with mids, even if Abyss claims it's all perfect. However, I still want to hear what Abyss guys have to say. I would also like to hear Audeze engineers, and Hifiman, and Focal, it's just they decided not to share their opinions so openly. I would respect them all equally, and again, take all of them with a grain of salt.