Sound issue
Yea you would leave your rears as is, then the amp literally wires up in between the factory amp and speakers...speaker line level going in and out
Those RMS ratings are for a sub in a properly designed enclosure, they can't handle as much when in a free air application like you've got.
That Alpine MRP-F300 would give you 75w's RMS x4 @2 ohms, plenty for it to be loud and sound good on both your front speakers and subs
Sorry, I don't understand what you're trying to convey here. Audio signal is either speaker level (amplified) or line level (pre-amp)... it can't be both. For most aftermarket amps you'll have to also get an aftermarket head unit since most amps accept only line level input (RCA) not speaker level.
I meant speaker level (amplified) my bad. That Alpine ive mentioned here several times has a built in hi to low.







