ive been reading and researching about ground loops and such for a few days now, and i have to say i have found some very interesting stuff!
while reading on a ham forum, i ran across a guy who has been dealing with RF grounds and mobile installations his whole working life.
he said the same thing you did about running the negative wire of the radio right to the battery terminal, but not running the amp's negative to the terminal.
not sure if you know the "why", but he said its because the radio's negative lead is isolated from the chassis, so its ok to go to the terminal.
however, with the amp, (or most ham transceivers), the negative lead is tied to the chassis, and therefore needs to be at the same potential as the shield of the antenna coax.
therefore, connecting the negative lead of the amp to the vehicle body puts it at the same potential as the antenna coax shield.
makes sense to me!
learning alot, and just wanted to let you know i found someone "in the know" about this stuff who agreed with you.
cant wait to get the amp and try some of this stuff!