Question regarding factory Head Unit

I just installed a 4 channel amp on my truck along with some jbl 609c speakers front and rear. The amp was a pioneer DX874 its supposed to get close to 100 watts rms per speaker. I adjusted the gains using an oscilloscope and a 1k test tone. First i maxed the volume on the headunit to see where it started to clip. Then i bumped the gain on the amp. Im only seeing 15 volts per speaker before it starts to clip. Will the factory head not allow me to get max rms out of the amp? Or am i doing somwthing wrong?

 

To start we need to know the year and what you have now...JBL, non-JB, etc.

 

21 tundra crew max non jbl variant i got the pioneer powering my door speakers and i have a skar 1200 watt powering my 2 10 inch subs

 

Nvm i think i figured it out. I didnt do the formula, 90 watts rms x 3 ohm = 270. Square root of 270 is 16.43 volts. I was getting 15 so i can prob adjust a lil more.

 

Check out my system build thread. The link is in my sig below. My advice is get rid of the sub par factory head unit.