A really off the wall idea...
Is the power to your new iMac (Congratulations!) "conditioned" through a UPS or any other type of 'filtering' system?
If not, could it be that the circuitry in the new iMac is more susceptible to 'ebb and flow' of the power in your house and may be passing this on to the speakers.
Just a S.W.A.G. on my part..
Best of luck in finding a solution.
Bob K. rnkiii