The Calais version has a pair of 8" speakers in the rear shelf, although I must admit I've never heard them working. I have however insalled 8" subs in the shelf a few times in VT Comms, you just need to make an MDF spacer and get the grills from GM spares. They sound OK but I reckon a 10 or 12" sub in a box would sound heaps better than going to the trouble of trying to get them in the shelf. Plus the fact that you wont find too many good quality 8" subs any more.
If you really want to do it try Pioneer or Crossfire BMF or Jaycar speakers, there may be some cheaper stuff around in 8" , but as Will Smith said in Men in Black ....... Daaamn.
Mike