I have 2001 Toyota Corolla with factory Radio/cassete player. I was
planning to add a subwoofer in to get more bass, and came across three
options.

1) Add a powered subwoffer tube like Bazooka,
2) Install a single sub along with an amp
3) Replace the factory speakers with good speakers that may add some
bass

I would not like to replace the factory radio, as I also plan to add a
CD/MP3 changer and the factory radio has an option to connect CD
changer to it. But unfortunately the factory radio does not have any
RCA out.

The harness has output for two front speakers and two rear speakers.

As I do not have experience in this area, I'll appreciate if someone
can let me know my options. Some questions that are popping in my head
are... If I use a bazooka sub, where will I get the audio signal
from?, where will I connect the amp, as adding an amp to existing amp
will also ampify the distortion.

Thanks,
Vikas