I have a Pdx 5 Amp but i dont have any subs I was wondering if it would make a big difference with the sound quality if i had the amp installed or not. with just speakers and no sub woofer.

I have aftermarket speakers as well.

 

It should improve the sound quality and make them louder if you hook up the amp. It's not going to make your sound system earth shattering but it will be an improvement. The quality of your aftermarket speakers also will play a big role in the improvement you see.