OEM head unit, aftermarket speakers..
 

Here is one scenario I was unable to find any information on via search.
I would like to keep the OEM head unit for simplicity and to prevent a smash/grab. Also not trying to spend a ton of $$. Bear with me, I'm somewhat of an audiotard.

The performance of the audio system is lackluster, no definition, sounds like it's playing through wax paper. Not muddy or blown, just sloppy and not very good. In searches I found that the OEM amp sort of sucks to. Here is what I would like to do.

OEM BOSE head unit and amp.
mid grade aftermarket speakers.

Ultimately I would like to just replace speakers and get some clarity. Can it sound decent with just upgraded speakers of appropriate wattage. Not expecting audiophile or competition grade performance, just want to get it sounding decent with some clarity. Is the system able to do that with the OEM amp and head unit?

Has anyone tried this and what were the results? Also any guidance on some middle of the road, Pioneer-esque replacement speakers, and amp if really necessary.

Thanks much.