Subwoofer upgrade? (in stock location)

[cymro]

I have the upgraded HK system (logic 7) but finding the subwoofer 'weak'

(I love Jazz music - not a boom box type)

There are some upgraded shallow mount subs that will fit in the tailgate in the stock location:
Pioneer and Blaupunkt have some. Has anyone swapped out and tried powering them with a second amplifier to get something out of them?

thanks
E