POP sound when turning off TT

Equip:
TT: Rega Planar 6
Phono Preamp: Lehmann Audio Black Box SE
Integrated Amp: Cambridge Audio CAX81
Speakers: Monitor Audio Silver 300

I just swapped out the phono preamp with the Schitt Mani that I had.  I immediately noticed the humming/buzzing sound was gone when absent any source.  Probably a grounding issue.

However, now when I go to power off the Rega (it comes with a separate PSU), I get a loud POP out of the speakers.  Keep in mind the phono preamp is still on.

When I had the Schiit, albeit a much less quality preamp, I never got the pop. 

What can/should I do?  Ground the phono preamp to the CAX81?