A basic power distribution block is used to split a power wire coming off the battery into several smaller power cables. ie-one 4 awg cable pslit into 2 8 awg cables. If you have multiple amps and/or electronic crossovers a block will come in handy when wiring it all up.
Also they make more advanced(more expensive)blocks which can have built in circuit brakers and other such fun stuff
The preamp outputs on your amp are used for connecting several amps together to achieve more power output.
Never used bass blockers before have always used crossovers but i do know that they act like lp crossovers so whatever Hz the bass blocker is rated for is the lowest freq. which can be passed through to your speakers
Eclipse 8445|Alpine Type X 137r|Diamond D5 300.2/1200.1|ID 10v.3|
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