sub obviously has some very little treble sound coming out from it ....

the amp adjust the frequency ... by smoothing (e.g rounded up) the pitch for the woofer bass ... but since it's rounded up ... some might not be fully rounded up ...... good amps or double amp (rounded up then rounded up again) can facilitate better bass

pls excuse me if i explained wrongly .... this is what i remember from my fiziks .... it's been more than 10 years ... lol