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
Spare Tire Mount Subwoofer / Soundbox - Sold by Volkswagen | VW GTI MKVI Forum / VW Golf R Forum / VW Golf MKVI Forum / VW GTI Forum - Golfmk6.com