2004 Quest SL Radio Crackle

The radio in my 2004 Quest SL has a crackle sound out of all of the speakers once the volume hits 18 or more. It is very annoying especially when you are on the highway and need some extra volume. This happens with both the radio and CD playback. I have the standard 8-speaker stereo with no satellite and no DVD player. The dealer has replaced the head unit and the both sets of speakers in the back (Near the bucket seats and in the hatch). The issue continues. I was just wondering if others have this issue. The dealer says that the stock speakers cannot handle the power that the head unit puts out. He mentioned that other Quest owners have complained about the same problem. The only way I can resolve this issue is to purchase aftermarket speakers. This does not seem right to me. Have others experienced this issue?