1) Valve amps should not be left on since the life dependancy of the tubes
is affected.
2) Class A amps should not be left on due to the wasted energy and heat
that they generate.
3) The rest depends on whether your amp has an inrush current limiter or not.
- If it does (your lights do NOT dim when you switch it on), then turning it
off would be ok.
- If it does NOT however (your lights DO dim when you switch it on) the
amp should be left ON all the time since considerable stress occurs at power up.