I dont think it would be a bad idea for you to go with the fi 15 sp4. i just installed mine last night in my altima and it sounds incredible by itself.
Its the same way with my SAZ-1500v2. it doesnt get loud till you get to high levels and its turned to the max with no clipping as well. and im using a Fi Q 15.
Swift has 3 18 sp4's now and i went to a comp with him last weekend. the sound quality is pretty shocking...if your just going for sound quality get a custom sealed box for it and it would be sweet. my car is like yours..im putting a 15" sp4 in a 4th order. these subs are perfect in my opinion.