I've had an issue with the H/K upgraded sub in my W211 since new and I can't confirm if this is because it's faulty or just weak. It happens when I play it at full volume on particularly bassy tracks but the bass level is flat, not boosted. In my view the standard H/K upgrade should handle this but it doesn't and it distorts. Therefore I have to reduce the bass or volume which is pretty poor. I've been in contact with the dealer and they've had a listen and agrees it happens but MB say it's because I play it too loud and won't do anything about it!! I'm currently waiting for them to get another W211 in stock with the same setup so we can see if it happens on that too but as yet, they don't have one (or have forgotten, which is my guess). I just wanted to see if anyone else has experienced this? I suspect not everyone plays it that loud so I may be in the minority?