If you have a subwoofer it is your source of low frequency. What makes a good system sound good is isolation from speaker to speaker. Your sub will produce the lows, and you woofers will produce mids, and you tweeters will produce highs. You dont want your door speakers trying to make bass if you have a subwoofer. If they wont fit, find some quality speakers that will, and let your subwoofer do the work on the bass. Low frequencies are longer wave lengths so they take much more power and cone space to move the air enough to properly reproduce them. Idealy you want your door speakers making 0 bass, they can put much more clarity and force to the frequencies they are more suited to produce that way.