I just finished installing the JL Audio 6W0 (4 Ohm) 6.5" subwoofers in the rear hatch of my GTS. As was stated above, they were a direct drop in replacement. No drilling, no cutting...NO NOTHING.
So far, these are the only components in the factory system that I have upgraded (still have the stock amp, headunit, front speakers).
I'm no audiophile, but I can definitely say they are a significant improvement over the old stock rears. A pleasing low end punch. Certainly not the low bass of a 10" or 12" subwoofer, but pretty damn good for their size. Hey, for $200 bucks for the pair, I give it a
.