shrimpkabobs

I have a 1998 GS400 with the Nakimichi sound system. The previous owner installed an aftermarket headunit but left everything else stock. Since I bought it, I installed 200w 6.5 Pioneer door speakers both front and rear, and they get distorted at high volumes. Could it be because of the stock amp? I’m not sure how much it could output. I’m also running an aftermarket sub + amp as well.