The Bose sub hangs under the parcel shelf. Plenty of after-market installs are in the boot and there's not much in the way of sound deadening in the rear seat backs, so should be good.
I want to upgrade the Bose sub to a small JL Audio one (built into a housing that'll be slotted in the gap behind the arch). Will also maybe look into upgrading the front components to Focal units, but then I'm told the head unit driving it all becomes the weak link and I don't want to go non-OEM up front.