kicker cvt12s & factory radio help

heyy i was just wondering if anybody else has tried these speakers out. I have two in the back of the truck powered by the zx850.2 but im not extremely happy with the outcome, i thought the bass wud be more powerful. The guy that did the installation told me that because its a factory radio the stock speakers nd subs arent playing as loud as they can and their is some kind of thing i forgot wat its called that can be installed into the factory radio that wud let them play at their full potential, i think it was a converter or sumthin i just cant remember. any help wud be great, thanks