Yeah, but that's not the argument. The concensus here is that a
Sparkomatic
or Rocktron amp sounds and performs every bit as good as any Zapco, Brax,
PPI, Xtant or anything else. I disagree.

Why do you disagree? I haven't been able to pin down your reasoning for
this aside from price considerations. If it sounds better, then presumably
you're implying that there's a significant amount of distortion or noise in
the amplifiers that you don't find favorable. While it may be true that
some amps are more prone to ground loops than others, I've yet to encounter
an amplifier on the market today where the noise hasn't been able to be
overcome or that introduces a significant amount of distortion.

If it were even remotely true,
everyone would just go to Auto Zone or WalMart to buy $49 Sparkomatic amps
and there would be no car audio stores.

It is true, and the reason that people don't do that is because other
factors are at play. Namely, reliability, warranty, customer support,
looks, features, and some people like to have a name for "irrational"
reasons (myself included - at one point in an old car I spent extra on an
ESX amp to run my component set just so I could have all ESX amps - I
thought it looked nicer that way).