Klipsch ProMedia 2.1 speakers are tinny and distorted.

Hi,

I seem to be limited to connecting these speakers via the headphone jack on the back of my iMac but the sound is terrible and the speakers are new, as is the computer. Is there a better way to connect them? Am I missing something? I know these are quality speakers as I have an identical set on a Windows machine and they sound great.

Any ideas would be appreciated. Thanks.