I have a 16 with the non HK system, All i've done is replace the front speakers with a set of focal 4" speakers, ICU 100's which are efficient enough for the stock amp, on a set of speaker rings that i printed from a file on thingiverse. I grabbed speaker adapters off ebay and i put a JL 10w3 in a .60 cu.ft box running on an alpine PDX 600.1 in the trunk with the technic sub out cable and it woke the system up significantly. I still have the center hooked up, but i plan to disconnect it to see how it sounds once the new speakers break in up front.
I didn't do the component setup as i plan to migrate these speakers to the back shelf eventually, but seriously, it helped out a ton. The trunk sub is a requirement IMO and i plan to get the fitted box from visual ice on Ebay UK to make the trunk more useful, right now i pull the box to haul larger amounts of cargo.
Speakers:
https://www.crutchfield.com/p_091ICU...00.html?tp=102
Speaker Rings:
https://www.thingiverse.com/thing:40.../thing:4075904
Wiring Adapter:
https://www.ebay.com/itm/Y-Harness-F...EAAOSw~K9dza0u