Should I just turn the volumn down on the preamp before I shut the power amp off?
Yep.... And if the preamp has a mute feature, mute the preamp before turning off the power amp.
Post back with your results.
I would conclude amps differ in their behavior when powering off. Case in point: I have had 2 amps from Belles, the first was the HotRod 150. It never cared about sequence when powering off.---The 150a ref. I now have requires the pre off first---or change to an unused input,by remote,in my case.---If it ain't music,I don't want my amp reproducing it.