iMac using VBL Spot speaker

Hi

I am using the nice VBL Spot speakers with my iMac. They're connected through the headphone jack on the iMac. Everytime I switch the iMac on and off the speakers send a cracking noise over the speaker. It sounds to me like static charging and discharging. Does someboday else have the same issue or even better, has someone a solution for that? Harmann Kardon won't help. Tried that one already.

Thanks

Michael