[chuchito]

If I understand you correctly, you installed these speakers in homemade boxes under the seat connected directly to the radio? If so, there are a couple of issues here.

First, if these speakers are not subwoofers, they should not be installed under the seat. Only speakers that play low notes or bass sound good under seats, in the trunk, etc.
Midrange speakers and tweeters should be mounted where they have a direct path to you such as in the door panels, dash, or kick panels.

Second, if they are subwoofers, your radio does not have enough power to play them. You need an amplifier for those.