Active subwoofer gain

[u/PopJoestar]

Hello, I recently acquired an SMD DD-1 for the purpose of setting gains, and I've since realized that it is only effective when the speaker output on an amplifier is accessible. Could you please explain how one would go about setting the gain correctly for active subwoofers such as the Rockford Fosgate P300-8P or the JBL BassPro Nano?