I have replaced all of the Shaker components in including the head unit. The Shaker 6x8s and subs actually are not that bad. The OEM subs/speakers are manufactured by Rockford Fostgate for Ford. An aftermarket HU will allow to adjust the crossover frequencies to the OEM speakers and dial them in better. However, you will reach the limits of the stock speakers quickly with a good HU. I went with a pioneer AVIC nav unit with Infinity reference speakers. JL Audio makes a costume fit sub box for the trunk in S197 Mustang.
07 GT vert, Satin Silver, 5spd, premium trim, interior upgrade pkg, comfort group, GT appearance package, cloth top
ADDED: 302 Stroker, KB 2.8 Mammoth @ 20psi, DOB belt drive, E85 fed, TR6060, ATI, Cervini hood, Meziere, Pypes Long Tubes, O/R X pipe, FRPP GTA mufflers, 20" Bullitts, Eibach Springs, BMR relo brackets/UCA/LCA`s, FRPP 4.10, etc, etc.