My Sunfire MKII sub "pops" even when my receiver..

Plug it into a different outlet see if it changes. Are you using a power conditioner of any type? If no, try one. If you think it is receiver related when the unit is off try
disconnecting the interconnect to the sub and see if it does it. If it still does it is something in the A/C line

There was a thread a few days ago about Sunfire subs popping and humming.

I have had pops when the sub turns itself on and off. If your system has been off for a long time and the sub suddenly pops, this could be the sub turning itself on when it perceives a signal. Since your receiver is off, maybe something else like a power line spike is faking out the sub.

If your system has been playing recently, and you turned it off 10 or 20 minutes ago, then the pop could be the sub turning itself off. I observe with my subs that they pop themselves off if they sense no signal for a while.

Of course, while these are what I think might be pops associated with normal operation, you might have pops due to a malfunction.