Any Cure for

I'm running my iMac via left and right channel cable straight out the audio out connection into one of my receiver's line inputs and have noticed that every online radio feed I listen to through iTunes sounds tinny and as if it was being heard through about 20 feet of cardboard tubing (best description I can come up with). Is there any fix for this anywhere in preferences/settings/etc, etc or is this just the nature of the beast? All the stuff I've ripped from CD to iTunes sounds waaaay better than the streaming radio feeds...