Goal ko pa rin is pamodify to fix the turn-off pop sound.
Does the popping sound comes at the same time when powering off or a decaying popping sound?
Popping at the same time power off - most likely the problem is arcing power switch. The modification for this is to put 0.1 micro farad AC capacitor 630v like those ones used on electric fan. Two of these needed if the power switch is double pole.
Decaying popping sound is caused by the discharge of the psu filter capacitors specially if they are the big values. Not sure can be done with this.
As Sir V said, using switched power strip, you can try turning off from it if you have one to isolate if there is arcing on the amp power switch itself.