What is the differance between a subwoofer preamp output and front/rear preamp outputs on a HU?
This HU has the subwoofer outputs https://www.onlinecarstereo.com/CarAudio/ProductDetail.aspx?Productid=13657
This one doesnt https://www.onlinecarstereo.com/CarAudio/ProductDetail.aspx?Productid=13239
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Well, you won't have to buy an external crossover or split the outputs of one of the set of channels if you have a sub output and front and rear outputs, and you want to use a 4 channel amp and a sub amp. Also the head-unit with a sub output probobly has a sub level control. So basically the cd player with sub outputs is more versatile.
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The cd player with a sub output allows more flexibility in system expansion. As was pinted out above most cd players with a sub preout have level control. This level control is a separate and indipendent volume control that affects only the subwoofer. Thus if you have way too much bass at a low volume level, you can crank the ol subs down a notch or two without touching the main volume control. It also allows you to keep the bass setting on the cd player at the nominal setting, no extra, no less. If you want to add subs, get a cd player with sub preout and level control if you can.
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Top Secret, I can tell you but then my wife will kill me.