This is just my experience in my 89 S-10 ext cab. I have a Focal polyglass
6.5" component set in my doors and was running a Phoenix Gold 400ZxTi on them. Sounded great. I recently bought a Zapco reference 350 to replace the Phoenix amp (it matches my Zapco ref 750 on my subs) and was really surprised when I listened to it. Voices seem clearer, more defined, also I can play music way louder and it just sounds deadly. I had to turn the gain up on my sub amp to keep up. The music is just easier to listen to. I always thought the zxti was a pretty darn good amp but the zapco makes it sound, well, mediocre. I'm loving it.