Pangburn

Help - I have no idea what could be happening with my stock non-BOSE stereo (I'm not knowledgable with steroe stuff). I get a ton of distortion from primarily the left front speakers - both of which the factory replaced once and the problem went away for a while but its back. A local shop took it in and it didn't do it for them so they couldn't help. Its a Bass thing, if I turn bass off through the HU the distortion goes away. The right speakers don't have this problem. It doesn't happen all of the time so I don't think its the speaker. I don't want to loose the steering wheel controls so I want to keep the stock HU.

Any chance its a ground issue? Other suggestions?