This.
Honestly, if you bought a Cayman/Boxster to listen to high quality audio, you bought the wrong car. The Bose system is more than adequate and the Burmeister is a total waste. These cars are loud rowdy and raucous... There is usually too much engine, tire, and wind noise to really enjoy a high fidelity audio experience. I suppose the argument can be made that if you use your 981 as a DD and just want to have nice tunes while sitting in traffic, then OK...
As others have mentioned, the proper way to do a subwoofer is to place it in an enclosure where it can move the necessary volume of air (usually in the trunk is the least obtrusive place you can put one). Take it from someone who used to rock a pair of Kicker 10" comps and a 500 watt MTX amp in all my previous cars (except the BMW).
You don't want to half-*** a car stereo, and a 981 is simply a bad platform for that. Enjoy the music of the road and push that pedal on the right a bit more.