*disclaimer* didn't watch the video
You need a digital multimeter (DMM)
info in this thread is accurate
http://www.ft86club.com/forums/showthread.php?t=42239
I assume you have a line out converter in this setup, although with some amps they will take speaker level, so I could see that too.. eitherway you can use the DMM to check your signal by using the balance function on the head unit and reading the outputs
once signals good set gain using DMM
follow this:
http://knowledge.sonicelectronix.com...l-multi-meter/
get test tones free here:
http://www.ronelmm.com/tones/
I use the 60hz and 80hz for tuning my sub, I just use BT off my phone,
unhook sub from amp, hook up DMM, set HU volume 75%, play test tone, set gain.
after following these steps report back?
also, good luck and feel free to ask more questions, the above is the super cliff notes version of a lot of research on my part.