alpine cva-1003 rca's

The output can be set to either a full-range signal or a low-pass, level/phase adjustable subwoofer-type signal.   What you'lll do is make sure it's set to the full-range output, and then just remember that if/when you fade the volume to the front of the vehicle it's going to affect the volume for the entire system (since the preamp outputs are only for the rear channels).

here's the brief instructions in case you don't have the user's manual anymore

1.  press and hold the setup button for 2 seconds

2. press the preset 6 button to set to the "SYSTEM" setting mode.  Press the preset 6 button again to select the "SUB W." mode

3.  Press the DWN or UP button to select subwoofer on to off

4.  Press the setup button to return to the normal mode.

Actually I'm almost positive the Alpine 4-channel amplifier will have a pair of full-range preamp outputs, which you would connect to the DEI amp's RCA inputs.

Since your head unit only has one set of preamp outputs, you would normally need to use a y-adapter for each of the two input channels coming from the head unit,  to give all 4 channels of the amplifier signal inputs.  However, this alpine amp has an input selector switch which lets you use just one set of RCA inputs and still have all 4 channels being output.  You lose the ability to fade front to rear but that's not really that big of a deal in my opinion.

Because both amplifiers will be getting a full-range signal input, you'll just need to make sure you adjust both amps' crossover settings accordingly.