I've replaced my door speakers with Infinity reference 6x8" speakers. They really brought a more "crisp" sound to the front. Mid and High ranges are clear without being sharp and piercing. I also have a 10" pioneer sub in the trunk nestled in the pocket behind the R/H wheel well powered by a rockford fosgate punch amp. It really doesn't take up much room at all and adds just enough of the low sounds to balance it all out.
I honestly haven't changed out the deck speakers.
They do have something of a filter to the rear deck. A lot of the guys who switch to the shelby kicker audio upgrade kit really noticed this (which is the same 6x8 in front as it is in back). The Shaker 500 does limit the power in the back and putting speakers back there that require more power to drive them make them seem like you have to turn the fade more to the rear to get sound out of them.
If you go for the easy to install clean look for a sub in the trunk.... It's the amp and sub as a kit without the rest of the shelby kicker kit. Kicker Audio 10 Inch Woofer
Just something I noticed, even if you turn the fade all the way to the rear, the same bass level can be heard from the 8" door subs. Just something to think about when you are trying to balance out your system.
2011 3.7L MT82 Kona Blue GT/CS Pedestal Spoiler, Cervini's HE hood, Stillen chin spoiler, BBK headers, GT T/B, Steeda tune, Ram 9.5 Dual Disc clutch, 3.15 axle T2R Torsen, 285/35/18 w/ Steeda 18"x 9.5"+44mm Q1s, Steeda Sports springs/Koni struts/Viking shocks, Shelby adj front sway bar, Boss302 Rear sway bar, Whiteline Watts Link, 14" "Brembo-ish" brakes and 3/4" front wheel spacers/ARP studs.