Is your sub box sealed or ported? Pics would help.You have just 4ga supply wiring? That amp already has a 100watt onboard fuse. Id run the amp you have, Remove the rear deck speakers and wiring from those. Spend the money on this sub and put it in a 2.0cu ft enclosure tuned to 32 htz sub rear port rear. Leave everything else alone for now,Sounds like you want more bass.This sub in the enclosure will Tank all over those two subs combined,Your car will flex pretty good and will fill the cabin space pretty nicely with those rear deck speakers out. Other than that.. Be time to do some electrical upgrades to keep up with adding another amp/amps and more cost.
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Check the measurements to make sure you can get this enclosure to fit in the trunk. Min tools to put these together, you can do it yourself in one weekend
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That is what I would do.. and to use what you have now and save for a better planed out system