Hmmmm. Interesting, but it's pretty obvious that they've put little time into dialing in most of those amps. THe Mesa MkV and the Bogner ALchemist in particular both sound truly atrocious, and I know those amps are both capable of sounding very nice.
Also, playing classic rock riffs through real high gain amps is not really showing what they really do.
It has some worth though. It's good of people to put up any comparative clips like that.