From JL( or link )http://mobile.jlaudio.com/support_pages.php?page_id=143

Necessary Equipment
• AC Voltmeter (Digital display recommended)
• CD with a sine-wave test tone recorded at 0dB reference level in the frequency range to be amplified (ex. 50Hz for a subwoofer amplifier, 1 KHz for a midrange application). Do not use attenuated test tones (-10dB, -20dB, etc.).

The Nine-Step Procedure
Step 1: Disconnect the speaker(s) from the amplifier.

Step 2: Turn “Off ” all processing on the head unit and the amplifier (bass/treble, loudness, EQ, etc.).

Step 3: Turn the input sensitivity control on the amplifier all the way down and switch “Input Voltage” to “Low”.

Step 4: Set head unit volume to 3/4 of full volume.
This will allow for reasonable gain overlap
with moderate clipping at full volume.

Step 5: Cross-reference the amplifier model used and impedance load per channel …..below( i did this for you) to determine the target output voltage.
Note: When bridging two channels, the impedance each channel works at will be one-half of the load impedance. Therefore, it is necessary to divide the actual load impedance in half and use this impedance in the chart when bridging two channels. Also, the voltage found in the chart should be doubled.

Step 6: Verify that you disconnected the speakers before proceeding. Play a track with an appropriate sine wave (within the frequency range to be amplified) at 3/4 head unit volume.

Step 7: Connect the AC voltmeter to the speaker output of the amplifier.

Step 8: Increase the input sensitivity control until the desired voltage (determined in Step 5) is delivered. If multiple subwoofer amps are being used, set each one to the same exact voltage and you have also level matched them. If excessive voltage is read with the control at minimum (full counterclockwise), switch the “Input Voltage” to “High” and re-adjust.

Step 9: Once you have adjusted each amp to its maximum unclipped output level, reconnect all the speakers and proceed to adjust the level balance between the subwoofer and satellite amplifiers by turning DOWN the input sensitivity controls of amplifiers that are playing too loudly. Do NOT increase the input sensitivity of any amplifier as this will defeat the purpose of this procedure by permitting excessive clipping (distortion).

Target voltage for your amp 787W at 4ohms(as you listed) is 56.1Volts

setting xover freq…..tun to approx 80Hz and adjust to your liking in bass

free test tones @ 0DB downloads (I use 50hz)