Hello johnwisam,
When using component speakers, it is normal to connect the head unit or amp outputs directly to the woofers and use a "high-pass" filter to the tweeters. Most component speakers sold as a set will include the correct filter along with wiring diagrams.
The12volt.com web site shows examples of passive crossovers here.
Amps usually include built-in filters and a means of setting crossover frequencies according to the speakers attached. If the amp is connected to full-range speakers, it is set to "Full" or "BP". When used to drive subwoofer(s) only, the setting is most often "low-pass".
Most stock radios and aftermarket head units are relatively low power and just do not produce enough power output to drive subwoofers adequately. It takes more watts to move the larger cones and that usually means a separate amplifier.
Hope this helps.
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