I'm posting a link that may answer all you questions about specific non-ML speaker replacement. But, you still need to come up with what you want to accomplish in the end.
As a side note; I would NOT recommend placing a separate amp down stream from the head unit. Your asking for major problems that the head unit can not handle. The factory H.U. is about 100 watts output. Not enough to drive nails through your ears, but should be sufficient for normal use. The amp in the trunk (I have the same in my 2010 ES) does a great job of punching out the bass. In my opinion, the biggest problem with the standard system is it's lack of a 5/6 band equalizer.
Let us know how this thread works out.
Thanks for the input and info guys. I just basically want a lil more bass and loudness from my system. Not looking to spend a great deal of money on a bunch of unnecessary stuff. Just want an enhancement of the stock system with a lil more volume while maintaining clarity.