Yup, I recently put a powered sub in my car behind the driver's seat, it's made a heck of a difference to the sound. Still fiddling with the settings to get the best sound when on the move, it's amazing how what sounds good when you're sat in the garage with the engine off sounds rubbish when you're out on the move.
In hindsight (it's a wonderful thing!) rather than just put a pair of tweeters up front I should have gone for a pair of full range 4" speakers and made a couple of enclosures for them somewhere in the front footwell. I've still got the rear speaker connections available on my head unit though, as I've spliced the tweeters into the front speakers, so another mod is feasible.
