2001 toyota prius subs/speakers

Hi all. i have a 2001 Prius that has the stock speakers currently installed and I have been wanting for quite some time to get new speakers/subs. however the problem is i know nothing about subs/amps/speaker systems/stereos especially in a prius. And the person i currently share the car with (the owner) is ok with the idea as long as i do not replace the stereo or modify the shape/original body of the car I.E. if the subs are bigger than the original speaker size. can anyone please help me, im looking for a system under $2000 that would be able to provide me with a lot of bass, i do not know much about ohms or amps or frequecies that kind of thing and am hoping someone could possibly explain how they relate to the sound as a whole. so basically if anyone has had any experience or knows much about getting new speakers/subs in a prius or a 2001 prius specifically help would be much appreciated. and i am hoping to get higher end speakers that will be able to last and be able to output a lot of bass without blowing out or wearing down. so any info anyone could give would be much appreciated.