Silly Question.

What speakers sound the best in the doors?

I have upgraded my head unit for a dab Sony unit.

but I was wondering about increasing the lower frequencies. 

 

You probably need to add a sub-woofer with its own amp.

The door speakers - no matter the brand - won't give you any more bass. It's their size and the effective "cabinet" that they live in that limits LF extension.
Unless you are handy with things like tapping into existing wires and providing a fused power feed to the new unit, I'd talk to a local car radio installer (NB  AFAIK the radio may not have a pre-amp out, so you have to grab full speaker level through a small transformer)