Re your second question, what you want to prevent is transients that may be generated at turn-on or turn-off by the preamp from being applied to a powered-up power amp. If that were to happen the transients would then be amplified, possibly to the amplifier's full power capability, or beyond it into clipping, possibly damaging your speakers. Or, at the very least, damaging the nerves of any people or pets who may be in the vicinity.
Therefore, power amp on last and off first.
Regards,
-- Al
Therefore, power amp on last and off first.
Regards,
-- Al