I'm actually skipping the subs. I haven't looked under the seat, but I'm pretty sure there's no amp in my base XF.

The stock stereo works fine with the windows up. At 90+ mph with the windows down, it doesn't work so well. Looking at a better stereo with Bluetooth so I can play the tunes from my Droid on the stereo (no more skipping CD's). I'll replace the stock component speakers in the door with a better pair, add another set of 2 or 3 ways behind the seats (not subs). That may be fine. If not, I can add an amp and sub in the trunk later.