I hope you are not using your stock head unit? If you are, then that’s your problem... You need to do one or two of the fallowing:
Buy a head unit with RCA outs for a subwoofer..
Or buy a cross over to filter your sound, and you can normally boost your bass 12db around 40hz, you will also be able to filter out all high frequencies.
Also what is the exact model number on your sub? It was mentioned here that your sub is dual 2-ohm voice coil. That is not always true. Kicker make there comp subs with dual 2-ohm voice coils or dual 4-ohm voice coils. Putting a 2 ohm load into a 4 ohm sub will not allow your sub to function properly, and you will shorten the life span of your sub. So please post the exact model number of your sub.
Also the MTX 8302 is measured at 425 x 1 watts PEAK at 4-ohms (not 2-ohms, it only runs at 4 ohms in a bridged mode). That is the peak power, which means you will not always be getting 425 watts (the RMS rating is 300 watts).