The best/most aggressive sounding 4.0 that I've ever heard was a h-pipe setup, shorty headers, SLP Loudmouth 1s, & no cats.
Gives more of high pitched racing car growl than the GT roar, & honestly it's the only V6 I've heard that doesn't really sound like a V6.
However the setup, cost, & loss of back pressure without coutermeasures makes it impractical for most people.
As for a filter, like others have said go with AM or Ford Racing for your best options.
Auction sites & third party sales reps don't have the backing that sellers posses.
2006 Aerodynamics Prior Design Racing 4.0 S197
*APDR Conversion (Body & Engine Bay)* -FR Transmission Swap- *Auburrn Gear HP Series LSD* -FR 4.10 Gears- *Cervini Type IV RAS* -Bama X4/SF4- *FR One Piece Driveshaft* -FR Ceramic Headers- *Custom MPC 12C "Hidden" Concept Exhaust* -Z26 Brake Kit- *Steeda & SR Adjustable Control Arms* -HID CCFL Headlights/Gen 5 LED Tailights- ETC