Turning off tube preamp first sends solid state amp into protection?

I have been experimenting with using a vintage Luxman CL40 valve preamp as a separate preamp stage for my L-509X integrated. Because I leave the solid state amp on 24/7, I accidentally turned off the preamp while the 509X was still in Separate mode. The integrated immediately went into protection mode with the power light blinking. I powered it down into standby mode, then brought it back up and it remained in protection mode. It wasn't until I unplugged the 509X and plugged it back in that everything was cool. 

Did I do any potential damage here? What it it happens again? It's pretty easy to accidentally turn off the preamp when I'm leaving the 509X on all the time.
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Guy walks in to his doctor's office and says "Doctor it hurts when I do this." Doctor says "don't do that."  Henny Youngman
I follow the jeenam and imhififan order: 

ALWAYS power the amp on last!
ALWAYS power the amp off first!

It works for me.

L.O.F.O  - Last On First Off
Then turn amp off first! Like I do! Always use the KISS principle!,😁