There may be a write-up somewhere on here, but basically the system as pictured here was installed while the standard ST1 audio unit was still in the car (the Infinity speakers I had installed some time before that). I discovered what you now have discovered (i.e. the stock head unit amplifier in the ST1 sucks). I then added a little mini 4-channel Alpine amp that fixed things a bit, but the sound was still muddy overall and of course with limited bass, as there is no sub in the ST1. That's when I decided to get serious - I got a used ST2/3 Sony OEM sub box from EBay, took the driver out and stuffed it with Polyfill. I also upgraded the driver to a Rockford Fosgate unit. This is a decent bass solution - it can't go real low of course, since the box is just too small for that, but it is tight and clean and I wanted the OEM look anyway and was not prepared to sacrifice hatch space.
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The JL Audio system itself originally comprised the following (in order of signal flow):
FiX-86
TWK-88 (DSP)
XD700/5v2 (5-ch Amp)
This transforms the listening experience entirely. It really wakes the speakers and the sub up of course. But the most important factor in getting good sound is the DSP, which time aligns all the drivers so the sound arrives at your ears exactly when it should. The FiX-86 unit comes into its own here, as it removes all the OEM processing and frequency response nonsense that is built into the head unit and gives the DSP a clean flat line level signal to start with.
More recently, I was able to undertake a separate SYNC3 upgrade with Navigation (including a 2018 Focus SEL head unit/ACM, which gave me HD radio and also satellite radio, as well as CarPlay/Android Auto).
From this:
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To this:
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Through some research I discovered that the higher end head ACM units can be reprogrammed via Forscan to output a pure 4 channel flat line level signal directly, so I was able to remove the FiX-86 unit entirely, which both simplified the system and improved sound quality even more (the noise floor in this setup is basically dead silent now).
I did a few more subtle tweaks, such as a new Ford OEM shark fin satellite/nav antenna from the Ford Fusion (painted to match obviously) and an extra SYNC3 USB port in the console for the passenger:
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That's it - what is visible is all very much OEM looking and all the aftermarket equipment is hidden, although I couldn't resist a little JL Audio logo on the tweeter housings.....

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