Any really good amplifier will be able to handle full power
more-or-less continuously - certainly for several minutes - without a
problem.

Hmmm, yeah - I spend my life listening to square waves and pink noise so I
don't need any transient handling capability, but those who listen to real
music do!


If slam really isn't just another piece of marketing nonsense, then it
must have a reality based in a particular sound produced by an
amplifier - that would make it some sort of specialist device for
music production, not Hi Fi reproduction.

It's mostly marketing nonsense, and used a lot in the pro audio world where
transient response is vastly important, and multirail amps (class G or H I
can't remember) are more common. You should only hear a "slam" if a "slam"
was recorded or intended by the producer/artist/engineer.