There is such a thing as "clean" and "dirty" power. A perfect case in
point
would be to run your amplifier with a cheap battery charger as its B+
supply. You will hear a tremendous amount of "hum". In other words, it
is
"dirty" power. At the other extreme, hook up a whole bunch ( maybe 10 )
of
automotive batteries in series. Tap the "GND" from the connection between
battery 5 and battery 6. Tap the amplifiers output rails (+/-) from the
the top and bottom posts. to do this, you must eliminate the SMPS section
and some other items. The Rectifying diodes and the bulk rail capacitors
to be exact. You now have "clean" Power for your 400W/ch RMS amplifier.

Many things can affect just how "clean" an amplfier is. Power supply
topology, filter capacitors, Inductors, ferrite beads, transformer type,
board layout, star grounding, trace capacitance, trace inductance, PCB
layers ( e.g. 1, 2, 4 ). Do all these things right, and you have a
"clean"
power amplifier. Do any one or all these things wrong and you now have a
"dirty" power amplifier. There is even an extreme case where when the
output rails do not have enough bulk capacitance, the output feedback
circuit goes "open loop" for just a microsecond or so. In this time, the
output transistors modulate or "burst" at their peaks causing audible
distortion. Lots of cheap amplifiers will do this just before clipping at
full power.

Not all things are created equal. Power is one of them.

If you want a mechanical analog, how about a V-8 piston engine and a jet
turbine


That's all well and good, but the important thing to examine here is whether
the results are significant during normal operation. Hint: by significant,
I mean audible. While there are several ways to lower distortion (sometimes
at the expense of noise, or at the expense of reliability, or at the expense
of other forms of distortion, and so forth), there is of course a threshold
at which further reduction is not warranted, nor is it necessarily
recommended since there's almost always a tradeoff in some manner or
another.

So your engine analog holds true, but if your application is to just drive
down a road, then either one will perform as well as the other.