The rear side panels have a fair amount of space that can be used to house some subs. Once the panels have been removed, you're left with this:

 

The standard 165mm rear 2-way speaker screws to the rear of the side panel with the magnet protruding through the lower hole. This hole is large enough to accommodate some 10" subs.

I've created a sealed enclosure with dynamat by creating a wall from behind the metalwork along the red line. All the surrounding metalwork was then dynamatted with the exception of the top of the sill and the top of the enclosure (base of the window frame). I then filled the enclosure with some wadding.

By attaching pieces of MDF to the lower opening I managed to build a mount to screw the sub-woofer to. With 10" subs, space is a little limited, so I only attached some of the top and bottom screw locations to arc shaped pieces of MDF. The gaps were then dynamatted to seal around the edge of the sub.

I relocated my existing Infinity 165mm speakers to the opening to the left of the red line. The side panel requires modifying to enable these to be effective, although the gap between the top of the side panel and the side window does allow some sound to escape. Here's the installed speakers:

I recently got around to modifying the side panels:

This modification transforms the sound, as you might expect!

Refitting the side panels is quite tricky as it is necessary to cushion then so that they do not vibrate when the subs are working. I'm also going to try and put foam between the outside edge of the subs to seal against the rear of the side panels.