I agree with a couple of people here, ignore the 1st responder unless you want to go backwards in sound quality.
I also agree with others to try a different source. Also, try different speaker cables, even some cheap 16/14 gauge wire just to see if the distortion goes away.
Do you have any friends that you can borrow a pair of speakers or do you have another set of speakers that you can hook up to see if the distortion is still there? In the end, if it is the speakers, there are many different speakers in that range of expense that would be better than what you have IMO:
Totem Winds, Usher TD10 or TD20, Revel Studio 2