I have seen tube amps recommended for soundstage and musical separation. I've also seen them recommended for neutrality and a clean signal, so who knows how much that means
Fiio products don't have perfect specs, but they have good enough specs.
Unfortunately subjective impressions on the "naturalness" and "openness" of the sound don't mean much. Everything you hear is measurable, it just isn't always measured. You want to prove to me that multi-thousand dollar amps sound better than $100 amps when both sport inaudible levels of distortion, noise, rolloff, and have adequate power and slew rate, find a study that shows it with numbers and graphs.
You can't possibly think it's impossible to model the performance of an electrical amplifier, with all the decades of advanced physics under our belts.
As to your last question, I think it's a healthy combination of biases and hive-mind mentality.