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I know this has nothing to do with your original question, but to me flowmasters make a bad resonance sound at several different RPM ranges, and a magnaflow muffler doesn't, and sounds better in general.
In the performance series, the 70 series requires normal speakers and stock audio. The lower the number you go the larger the speakers and more powerful amplifier needed to overcome the resonance to the cab. At some point you will also need wireless headphones with microphone to speak with other persons in the cab.
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Slideshow: Called the Fortress, the 850-horsepower pickup combines Raptor underpinnings with military-inspired features, survival equipment, and a starting price of $285,000.