Personally, I would just replace the amp, etc. One of the issues is that while the Audiophile system may be a 5 channel amp, it does kind of route a significant amount of power to the sub so that it meets that requirement. Problem is whether or not it will handle the impedance of the speakers you may want to purchase. Many factory amps are not known for matching the lower impedances of many aftermarket speakers. If it can't handle a lower impedance, you may wind up having thermal issues with the stock amp.
I also thought about getting a 5 channel amp but many of them are physically too big which would wind up compromising my stealth install and utility. Therefore, I am thinking about a 300w 4 ch amp and a 250w 1 ch amp for the sub. I may also consider a capacitor because of the HIDs (want to keep them at steady voltage).