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It is to help protect it, but in the same token it is also there to cut down on distortion. Having said that, that rule of thumb is for a second order crossover. You can switch to a third order and crossover later if the driver allows it. And although the rule is 18db down at FS, I try to be closer to 24db if I can and 20 minimum. But a lot of this will show up on your graphs when you take distortion measurements. It’s one of the main reasons you can’t just go for a linear response. You have to account for things like this.