amp turn off pop...

okay, got an amp turn-off pop question...since 'upgrading' my stereo at 100,000 miles, I now have an annoying amp turn-off pop. only on turn off.

I have the stock HU going thru an Audio Control low-voltage unit (converts the speaker levels down to RCA levels), then going to 2 amps. One is 4-channel for the four door speakers, the other runs my new fiberglass subs (2 8" punch running mono). they seem to be what is popping...more like thumping, upon turn off.

I have a relay on the turn-on lead on both amps...triggered by the Audio Control unit, which is triggered by an ACC from the Cig lighter (no 'amp' turn-on with the base stereo I have in an '04).

So, any other details needed to help me? Need some things to try to eliminate this. Ground issue? Bad trigger relay?

Thanks for any advice here!