For those of you who want more bass but don't want to sacrifice your spare tire or hatch space, I highly recommend this spare tire mount subwoofer : r/GolfGTI
2011 - 2017 VW Jetta Sub and Amp install to Factory Radio - YouTube
Focal Kit For Golf 6/Bora/Jetta MK6 09-14 - Includes Two Pairs Of IS165VW Component 6.5" Speakers - Creative Audio