OK, unless your Onkyo is DEFECTIVE or if you have it set up incorrectly, there is no reason your Polk's will sound worse than your PC speakers. I GUARANTEE it!
What exactly is it that doesn't sound right about it? They sound muffled? thin? tinny? boomy? crackling? Do they sound bad only at low volumes, high volumes? any volume? Do both speakers sound bad? Use the balance all the way to one side, or just disconnect the one speaker.