I'd check for dust accumulation. Try powering down the speakers for a couple of hours and then carefully vacuum both front and back stator panels. You can contact M-L as they are very helpful with recommendations. Good luck.
Thanks for the reponses. I broke out the vac and it significantlly decreased the hiss. I only hear it if I put my ear right next to the panel. Thanks all...OBTW anybody know the cost to replace the panels?