Cricoort,
I had the same problem and first thought my Bose speakers needed to be replaced. Then I tried listening with earphones : the same distortion on the bass.
I read all the above replies, went to Audiophile music sites, heard the exceptional sound quality difference from samples, then saw the prices. Ouch.
There was something bugging me: the sound quality had suddenly improved after making updates on my computer. I thought, happy, that I might have resolved my sound problem while doing all this without knowing it! 🙂 But the distortion came right back! 😟
I knew there was a way to have better sound quality with no distortion, but how??
I tried something different: I opened Window Media Player and it automatically updated my media library with all my purchases from the iTunes store. I chose a song that was particularly annoying because of distortion and played it with Window Media Player. Problem solved.
Next problem to solve: how to copy my zillion playlists from iTunes to Windows Media Player. 😟