What I believe is happening is that when you turn the power off the Positive and Negitive voltage becomes uneven as the PSU Capacitors discharge causeing a momentary DC Offset at the speaker which causes a slight speaker displacement which manifests it"s self as a small Pop.....
I have a guitar amp I just built which has a pretty Bad Tun on/off pop ,so what I did was added a switch at the back which disengages the speaker , I call it a "Standby switch" ... so I put my amp on standby which disengages the speaker , then I turn on the Power to the amp, then I turn the standby Off ,and I am ready to rock without any Pops ...I do the same but in reverce when turning the amp off... But my Pop was really Bad which I"m sure yours isn"t so you probably need to go through such steps to get rid of the pop...
Cheers