Wow! What a fight!
I'd agree this, IF an amp is not clipping (and that's a bigger if than you might think), IF the load is identical, IF the gain / loudness is identical (which it would be if the load was), IF it was conducted blindly, then it's feasible that a WELL designed amp might sound the same as another. However, WELL designed usually requires a bit of time and effort, which if the business is run properly means there's an associated price. The problem is that real world well designed speakers that are lower in coloration and distortion are more difficult to drive, this requiring more time and effort into designing and building an amp that is capable of doing so.
In truth, "hi fi" is in absolute terms pretty minimal gains for outrageous cost, and to our friends and family who are not quite so obsessive, we are alll "audiofools", even spending a a couple of £k on a system is for many people bonkers. Great, it's a hobby, a release, nothing more. If I want really serious music then I'll go and watch it or play it myself. Life is really too short to take this all quite so seriously! I never hear cyclists arguing that a cheap aluminium bike is as good as £££k carbon fibre model (though there is planety of inverse snobbery - envy perhaps?), I never hear guitar players trying to denounce Gibson and Les Paul as mere foo (other than a sad minority who being unable to ever afford to buy one will try to justify their plight in such a negative way). In contrast, large parts of this forum these days reads like a string of youtube comments.