Wow!!!! I'm having the same issue with bass distortion when the volume is turned up over 30! Last week I purchased a certified pre-owned 2013 ES350 with the Ultra Luxury Package. When listening to Beyonce's song Partition I noticed excessive bass distortion. To make sure the issue wasn't specific to this sound track, I listened to other music that has a lot of bass. The problem was replicated so I contacted the dealership. The subwoofer was replaced but the issue with bass distortion is still present. I'm going to call Lexus HQ today about the issue and if I don't like the response, I'll file a complaint with the Maryland Attorney General's office. It horrible that Lexus would continue selling cars with this type of problem and in my opinion, a recall should have been issued! Maybe the Lexus Owners should consider filing a class action lawsuit! The upgrade for ML is $4000.00 and I believe that Lexus should stand by their product!
Note: When I test drove the 2014 ES350 with Ultra Luxury Package, it didn't have this problem so I suspect that Lexus fixed the issue with the new model.