Finally got my back seats out, and the rear seatbelts. They are VERY heavy - took me some effort and a couple of pulled muscles to get the seats out of the car (coulda done with some help really).
Lots of noise from the back now, the exhaust is loud (and raspy and horrible). I need to find a satisfactory way to secure my sub to the floor. My initial test drive led to my sub, which was stuck to the carpet with heavy duty velcro, flying into the back of my seat, pulling the carpet with it. Obviously the carpet was held in place by the seats 
Took the sub out, and whilst I won't get to enjoy it's presence the car is a lot lighter. I'm estimating from the effort dragging it about 40kg, certainly heavier than an airplane-friendly suitcase. I managed to through it round a mini roundabout at a slight slide - it is a bit more nimble now.
Just the sub to secure, and perhaps some noise reduction.
Maybe I'll do as Guy has done, and put in a full-size spare on top of a wooden board, and fit my sub into the space-saver spare wheel space.