Adding a sub won't increase the volume of the other speakers but maybe you won't need to turn the volume up as high as you are now trying to get the sound you want?
Shelby has a nice plug and play Kicker 4.1 channel amp for the base system. The .1 gives you a signal for a sub. http://www.shelbystore.com/product-p...1-AMP2013B.htm
You can use a Shelby / Kicker powered sub or I think any other powered sub as well. http://www.shelbystore.com/product-p/KA-10-WOOFER.htm
Not a cheap system, but the plug and play aspects make installation a snap and the system sounds great.
2013 GT Premium Brembo
MGW, Blow Fish, AM Hood Struts
Tinted, Borla Touring AB, ProCal Tune
Full Shelby/Kicker Audio Upgrade