For the buzzing check to see if the tweeter has come loose. They snap into the fitting but I have come across a few where they were not snapped in all the way and that caused some buzz.

As for using aftermarket subs off that amp, don't head down that road as you will quickly find it is a different amplifier with different software.

Keep in mind a few things....

1. The door speakers are 2ohm and other than a few JBL and Infinity models don't expect to find that across most of the brands sold across the US.
2. The amplifier has an EQ system built into it that was set up for those drivers - changing the speakers will not undue that and may lead the system to sound worse than stock
3. If bass is what you are after without loosing space than I would suggest an Infinity Basslink powered sub. Its small, good sounding, and will work in harmony with your factory system.