The impedance has nothing to do with clarity. You just have to match the speaker impedance to the amp. The Sony amp uses low impedance speakers from the start and is not very powerful, so you need to use high efficiency low impedance speakers if you want any volume out of it. High quality components are usually 4 ohm and they typically need an aftermarket amp to sound decent. With a quality amp, you’ll get good power into 4 ohm, so you’re not so worried about max output per watt and low impedance. There are other technical reasons they use 4 ohms that involves inductor and capacitor values in passive crossovers and also dynamic headroom, amp power rail stiffness, voltages, etc. It gets really complex, and it really doesn’t matter for what you’re doing, anyway.
Before I built a full system, I just replaced the door speakers with 4 Infinity Kappa 86-cfx , and it made a huge difference with that alone. Replace the door speakers, and go with a good quality 10” amplified sub if you want some real bass, and it will sound worlds better. I’d highly recommend using foam baffles to seal against the door panel too. Try the doors first and see if you’re satisfied. You’ll get quite a bit of bass from the door speakers and you might not even need to mess with the deck subs.
If you want to add a sub amp, though, like lostneye said, if you tap the speaker outputs at the deck subs with a line converter, you’re gonna lose quite a bit of bass because the Sony amp rolls off bass the louder you crank it. There is still rolloff in the door speakers, but it’s not as bad. You don’t actually have to tap inside the door. The amp is behind the left trunk carpet and all the speaker wires go to there. I’ll attach a wiring diagram with the pins and colors.
The factory deck subs are 24.5 watts rated by the way and 2.3 ohms nominal impedance. The center connector is the one the speakers are hooked to, except the deck subs which hook to the blue one. This diagram is for the center connector.
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