DB1242DVC Setting the Crossovers

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2013 mazda 3

4 JBL GTO939
Alpine PDR-M65
Kenwood DMX4707S
Polk DB1242DVC

can anyone tell me the best cross over slop for this sub math says i set it right. but when i had it in a different car (stock headunit) sounded slightlyt crisper but i was setting through the amp. should be better through the headunit right? also if i have my jbl crossovers maybe be too high for the sub?