I wouldn't consider 2 Us amps de-3000s "daily driver" type of power, you'll have to have custom alternators (note the plural tense) and HUGE power cable to power two of them, not counting whatever amp you put on highs. As far as the subwoofers, I'd choose the X.X.X. for a little more musical application, for more SPL I'd choose the Ti Pro due to it's motor strength and huge mechanical excursion. Note that these really aren't daily driver subs either, I'd recommend two RE X.X.X. or Eclipse Titanium 12" subs for that, with about 1000 RMS each. SQ of either of these 18's isn't going to be that great, but choosing 18s I'd hope you know that. The 12s are more than capable of powering your SPL wants unless you plan to compete, and take MUCH less power.
4 X.X.X. 15s and 1 de-3000s (or 2 de1000s)would slam, it's just going to be so loud that you'll never use their full potential. They will work well and sound good doing it, as long as you are willing to accomodate the enormous current draw demands for those amps and also the space for the subs.