Several members have purchased the Boss bass 600, and have stated they will not fit under the seats, so I doubt the 1200 will.
I'm not an audiophile or anything, but I recently installed a pair of kicker DS650's in my smart with the stock radio 9, and I can step out of my car and watch these speaker rattle the outer door panels, and I can turn it up till my ears hurt. My question is, why would you need more bass than that?