The JL Audio XD400/4 (same as the M400/4) manual clearly states, as does the video, that no external adapter is required to run a high level BTL HU output into the amplifier. In that case you should avoid using an external and redundant step down device. I do not see a problem.
However, let's look for an exception just to be safe. Wire everything up according to the manual. Easy does it to begin with. If you experience alternator, ignition noise or switching transients of any type, a foreign noise, pop or distortion, as you elevate the volume level, or the input sensitivity control on the amplifier is not responsive enough, then disconnect everything and turn to an external device.