msi b350 tomahwk

i have msi b350 tomahwak and i'm using 2.1 speaker system (2 speakers and 1 subwoofer) which both speakers sound good but my subwoofer sounds bad.it's not amped and the bass is flat not boomy.both speakers connected to subwoofer by rca red white cable and my subwoofer connected to pc by 2 rca to...