Sound sounds very high pitched during dialogs

Hi ! I bought a Yamaha rx-v520rds (second hand) together with 5 speakers and a subwoofer. The guy Ibougjht it from told me everything works fine, all the speakers indeed work well (loud enough etc.). The subwoofer works really well and I ca' hear the bass clearly. But the main and central speakers (at the front) sound very high pitched during dialogs especially (all individually). I tried :

• reversing the wire polarities
• changing the sound processing modes
• changing the input channel
• different files
• different input devices
• tuning up/down the bass/trebble
• reading the whole manual (maybe I missed something?)

I am using an optical cable from the computer to the speakers and it worked fine on another system I tried a few days ago. But on this one, it sounds very high pitched. I also tried it with the previous owner's devices, and it sounded like that as well, he just told me "you have to do the settings for it to sound better"

Any idea what else I could do ? I wanted to factory reset the receiver but didn't find how...

The speakers are the "prism live 7.0"

Edit : each main speaker has 2 "sub speaker" (the circles) and only the small ones work. So the ones dedicated to lower frequencies don't produce any sound. No clue why tho

Edit 2 : it seems like the bigger sub speakers are dead...