JL Audio makes a Stealthbox that replaces the factory 8" subwoofer with a 10" sub. It's non intrusive and many members on this forum use it and said it greatly increased the quality of sound. I haven't heard one personally but have considered it. Here's a link to the stealthbox made specifically for our WK2.
Car Audio - Stealthbox® - Jeep - Grand Cherokee - Jeep Grand Cherokee '11-Up
As for amplification, your best bet is to get an aftermarket amplifier. Using the factory amplification will not do a larger, and more capable, sub any justice IMO. But that's not what you're looking for so I'm not sure how feasible it would be.
If you absolutely don't want an aftermarket amp, I believe some members have replaced the factory 8" sub with an aftermarket sub with decent results. You could search the forum...I've seen a post or two about this topic.
Good luck with you upgrade.