Ok so after a lot of research on this i finaly took all the advice I found here and this is what i did. My goal was to make the stereo in my JK sound like my bose system in my Sierra. I picked up a stock 8" infinity sub on ebay for $60. Then i picked up all 4 infiity 6.5" speakers from a JK on ebay for $55 and I left the stock tweeters in. Then I went to Walmart and found a $25, thats right $25, 200 watt amp to drive the sub. I thought I would need an amp to drive the 4 other speakers but with the sub dialed in the system sounds as good or better than my bose using just the head unit to drive the 4 speakers. Of course I peeled back the carpet in the back and yes the harness for the sub was right there. I traced the wires back and of course they where not hooked to anything. I spliced them close to the passenger seat and placed the amp under the seat. I had to get a line out converter from best buy and spliced into the rear speaker output right at the head unit. Ran rca cables down the dash and under the passenger seat to the amp. I mounted the sub with some well nuts in the pre punched holes and used 1/4 20 screws and large washers to hold everthing in. Budget Infinity

 

Well done!

Were the infinitys you got 2-way? How do the highs sound in the front?

I have the infinity system in my jeep, and i find the highs to be severely lacking. The bass is a little better than 'meh'... so i can deal with that. I'm thinking about replacing the fronts with either a good 2-way speaker, keeping the factory tweets, or a solid component set, replacing the tweets...

 

The 4 infinity 6.5 inch speakers are single stage. The sub is a dual voice coil so it gets both the right and left input. The bass is strong but with the aftermarket amp I have I can tone the sub down and reduce the bass out from the head unit which allows the front to take on the high end of the system. I dont know if the stock infinity system amp allows you to adjust the sub level so maybe that would something the factory could improve on.