If you run an amplifier that has a subsonic filter, you will get a nice improvement. The stock amplifier is gutless for any real power. I had 200w to the oem subs with proper cross over settings and they were amazingly better than with the stock amp.
i had an amp and 10' sub installed, with the stock subs disconnected.2 things that bothered me was it made the rear panel to the trunk unusable and there was just too much bass even on the lowest settings, removed the sub, reattached the stock subs to the amp and all is well.
Also a word of caution, my setup has the amp running the door speakers and two rear deck as well. The front tweeters and center speaker are off the stock amp. Meaning if you turn up the stock amp those speakers will become distorted. I scratched my head on this one until i realized they were still running off the stock amp and not the aftermarket one i had put in.
To be fair the guy who swapped out my speakers and put in the amp had to deal with the fact he had never worked on a G8 before so it was a learning experience for him as well.
I know this is an old thread, but I finally removed my sealed pioneer flat 12 and put the fosgate amp on the factory 8's. It does sound pretty decent. This at least made me want to replace the factory 8's with aftermarket free airs so I can keep my trunk space. I figured I would update since I started the thread.