Hade this problem with a -90s Ford, only difference is that I used a stock headunit, stock speakers cone was falling apart so bought some new coaxial speakers and plugged them in and it didn’t sound better at all, everything sounded “harsher” and you could really tell on the instrumental cymbals and other high frequency sound that it was distorting, was only the little midbass that existed that sounded a bit better since the cone wasn’t broken anymore but everything else sounded worse

only difference I found was that the stock speakers was 2ohm and the new ones 4ohm, I switched back to the stock ones with broken cones since they sounded better