difference, tweeters and component sets

what is the major difference between installing dome tweeters by splicing into existing speaker wires and component sets?

We need more information. 

I assume you're asking this: What is the difference between just adding a tweeter to existing full range speakers and a matched component set? Well sound quality is the biggest difference. Just adding a tweeter with a cap or simple passive high pass crossover to any old speaker is gonna yield unpredictable and often unpleasant results. Sometimes it sounds passable, most times not. This is why many stock sound system that use a separate mid and tweet sound like ass - since this is exactly what they consist of. Take a cheap paper mid and throw in a cheap plastic dome tweeter with a cap on it. Call it a "premium" sound package and charge and extra $600 for it. A good quality matched component set generally has a lot of research and engineering behind them. The woofers and tweeters are designed to work together to provide balanced, full frequency sound reproduction. The crossover network is designed to blend the two speakers as seamlessly as possible. It also takes into account impedance and phase corrections to allow the best sound and performance for those speakers. So to sum up: adding a tweeter to existing speakers = generally bad. Nice component set = generally good.

thanks alot man that explained alot

A little off topic and a very noobish question, but I've got a Pioneer TS-c160 component set but I'm missing one tweeter.. any recommendations on what I should use as a replacement?  Will any tweeters work with the existing crossover?  I'm using the pioneers in the back of my GS300 and Infinity Perfect 6.1 upfront powered by a SoundStream Vangogh VGA320.4 4channel amp with a Kenwood DNX7100 as my headunit.