What will an amp add?
 

I installed my head unit and a pair of db6501 speakers into the Z yesterday. Nice crisp highs, with a little midbass and very little bass. They are currently not amplified.

I have my eyes on a Polk 75w x 2 RMS amp. What will this change? More loudness? Better midbass? More bass?

I eventually plan on getting a powered sub of some sort, just trying to figure if spending more money on the db6501 speakers by adding an amp is going to really change much.

The amp will allow you to go higher in volume and will provide a fuller sound to your music. More bass is one of those pros.