Problem with subwoofers 15 inch?

I have 2 dual voice coil audiobahn 15s in a ported box powered by a kac 9105 d kenwood monoblock amplifier I know a little about subs ive been doing backyard hookups for at least 4 years and im stuck on why they wont hit certain lows its like they start too then the just make that dong noise so i kick it off right when i hear that ive heard i just dont have my lpf set right ive done little reseach but have not been able to figure this out wondering if someone could throw me a lifeline on this lol thanks