audio set-up for beginners [continued]

[mda]

Good evening mga sir, I recently upgraded the stock speaker of my Mazda 3 to focal 165AS front and 165SC rear, tanong ko lang, na notice ko lang kasi minsan yung sound humihina and then lalakas, ano kaya ang problema??? I plan to go back to where I have it installed but I want to hear from you guys muna baka alam nyo, thanks!!!

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Only at louder volumes? Baka stock head unit can't output enough power.