My Audio guy suggested it would be OK to do (in the UK). He recommended a focal active subwoofer that is 25cm. He raved about this but they are quite pricey for the power rating. I never auditioned this sub.
I ended up going for a flat pioneer sub driven by a separate amp - there's plenty of room in the boot for both with everything hidden (and wiring v simple as the battery is in the boot too). Makes a huge difference to SQ (this is not the Mark Levinson system though).