I would be stunned if turning off your power amp while the pre-amp is active would cause any kind of problems. Check with your power amp manufacturer if you want to be absolutely certain, but this is more-or-less the same setup I have (power amp + pre-pro + headphone amp) and there is no problem whatsover when my power amp is off or my headphone amp is off (they feed from a Y-split off the front LR pre-out).

See Mojo's signature