I sacrificed the left side trunk pocket where the jack& toolsare .......... to have a 8" W7 J&L sub installed (very neat & flush setup) as part of a system transfer / upgrade from my prior car.Anything larger than 8 inch is going to require major mods.

A good 8" sub is enough box space (sealed unit) to get the thump I wanted. Amps were mounted on bracket to the truck area topside ~ out of the way so you dont give up any normal trunk space. Depending on the power requirements of the overall system ~ I would also get a new battery.

Mark B
Massachusetts
00 A6 2.7T with chip & new diverter values
(Kenwood 7100 deck, Focal speakers 4 doors, J&L sub powered by J&L amps)