Full Range 20-20 signal?

[mcallister718]

This is the setup I plan on putting in my 2012 Ford Fiesta Sel sedan. The LCi6 should be able to pull the signal from the speaker level inputs and sum up the signal into a full 20Hz-20,000Hz signal. What I am worried about is that the signal coming from the 80 watt Stock head unit isn't full range. If someone could help me settle this question i would feel much better about the 1000 dollars I'm throwing into it.
AudioControl - LC6i - Line Output Converter

MRX-V60 - Alpine 5-Channel Sub-woofer Amplifier

Infinity 509CS 255W (Peak) 5-1/4 -Inch Two-Way Speaker Component System (Pair): In front and rear doors

JL AUDIO 12W3V3 SUBWOOFER 12" SVC SUB

I'm also worried about bass drop out with increased volume. Some stock head units drop off the lows at high volume in order to save the speakers from blowing. Does the fiesta utilize this feature?
Has anyone used the The Epicenter Plus™ by AudioControl to compensate for bass drop off if so?
How about the REQ5?