I have the factory sub hooked up to a Premier f90bt with the door speakers amplified. Although the factory sub sounds pretty good at moderate volume it gets overwhelmed at high volume. In other words it wont thump.
Also I get a poping sound from my sub when the car is turned on, My understanding is that there is a problem with the voltage and you need to install some sort of resistor in the line.
I have a 700BT that I just purchased. I bought the special harness so I can hook it up to my factory woofer for now. At some point I will get rid of the factory woofer and buy a large one that I can fit under the passenger seat. I was told the same as you though that it does sound alright, but doesn't have an earth shattering thump.