Andy Evans wrote:
If
you had 8 ohm speakers, putting the amp on the 4 ohm tap would be cleaner and
tighter, using the 15 ohm tap would be louder and a bit more ragged. Neither
will damage anything, and frankly you can set it at whatever sounds preferable
to your ears.

Not so. I clearly remember seeing an old valve amp go up in smoke when
the wrong tap was used.