The S550 Mustang offers an step-up 9-speaker premium audio system for premium model (200A/400A) Mustangs that while greatly improved over the base stereo and containing a 8+1 channel amplifier, lacks a subwoofer (or any wiring for one) that is included in the top-end Shaker Pro system.
Mustang6G member mikes2017gt has taken it upon himself to add a JL Audio Stealthbox subwoofer and sub amp to his 9-speaker setup. This installation required removing the panels in the trunk so Mike used the opportunity to add Dynamat Xtreme sound deadener to improve sound clarity and loudness from your stereo, as well as to keep out unwanted road noise.
If you’re a 2015-17 Mustang owner with the 9-speaker system and need a full walkthrough detailing his installation of a subwoofer and amp in a 2017 Mustang GT Premium, check out his DIY write-up.