Re: when i turn off the standby it makes a loud pop sound
Another reason to not use/have a SB Switch.
Try leaving it in the Off position, and just turn your amp on and off with the power switch.
Your SB Switch may be getting flashed pretty bad. You could try replacing it..... would not cost that much to see if it helps.
As others have said, I would imagine the choke is doing what it is designed to do, resist a change in current, and you are experiencing "inductive kick". It can seriously arc the switch.
Just a guess on my part.
Best