Set maximum undistorted volume on the head unit. This is NEEDED FIRST. Find out what it is. Make sure there are no speaker wires hooked up to any amps. From there with ALL the eq settings flat on the head unit and balance and fade centered... send that max volume, undistorted signal into the DSP which also should be "All Settings Flat on the EQ's" "All levels @ 0db and full up on the DSP's volume. You can set crossovers if you like after you set the gains. Just as starting points, try these. 80hz and down to the sub (low pass). Midbass set a bandpass say @ 80hz and 1500hz. Mids bandpass 1500hz to 4500hz. Tweeters 4500hz and up (high pass). With NO speaker wires hooked to any amplifiers send Sine waves (test tones)from the head unit, through the DSP to each specific amp so you can set their gains using a multimeter set to A/C voltage. Try a -10db 40hz tone for the sub amp. Try a -5db 400hz tone for the midbass channels. Try a -5db 2.5Khz tone for mids. Try -5db 4Khz for the tweeters.
You will need to know what power you want to send to each speaker though so you will know what voltage to look for on the speaker terminals. It's not hard, it just needs to be done in a specific order. Once the gains are set... turn the volume on the head unit down to a safe level. Open the app for the DSP... set the crossovers!!!! Very important to do this now so you don't blow up any tweeters... NOW hook up the speaker wires. Turn on some music and try out different crossover points and slopes as well as types... play with the EQ's. You are on your way at this point. You can also play with the time alignments of each speaker. Rabbit Hole.
To find out the voltage do this. Know the rms wattage of the speaker and it's Ohms rating. Take the power you want to send to the speaker i.e. 100w to the midbass. They are 4 ohm speakers. 100w x 4ohm gives you 400. Find the square root of 400. We know this to be 20. So we look for 20v at the speaker output terminals on the amp. This gets you pretty close to the power we are looking for. To get closer one should use an oscilloscope or a device designed to detect distortion. Power x Ohms squared = Volts. Any corrections needed, please correct me. This is as simplified as my brain see's this.