Or you can get an adapter from AndyMac which allows you to plug in a conventional Sub into the standard Bose (on the left side of the boot so plugs into the what the adapter on used to be plugged into the standard sub - obv you still have to run your own power to the car battery) system which just works of the bass setting on your head unit. I've done this and run a 8" Fli Sub (like mine more punchier than bassy) in the boot like this one:
FLI - FT10 800W Active Subwoofer & Enclosure: Amazon.co.uk: Electronics
Sure it eats up about 1/3 of the boot but not a huge amount and still plenty of room for 'boot' stuff... Makes the standard Bose Audi sub look ridiculous - even at normal setting just a fuller richer bass. I'd never go back in a million years - I run it using the standard OEM Bose speakers in the doors and it compliments it well. (if your speakers aren't knackered - have replaced 2 which started to rattle!)