I just read in the manual that if no RCAs are hooked up to Input B, the gain for Input B needs to be set the same as Input A. I'll go do that, and make sure A/B are both set to LPF at the same frequency. Will post back in a sec.
Put Y splitters on the two RCA input cables and run them to all four inputs, see if your startup pop goes away, worked for my friends crap Pyramid amp.
PLUS, something is VERY broken with your setup. The second video, Bass I love you, your sub should be MOVING like crazy almost the entire time. Your sub BARELY moves on the 29hz parts, but goes crazy at the 22hz parts! lol. Doesn't make sense. I'd almost guarantee that your voice coils are fighting each other! Either from getting a stereo signal, or one of them is wired in reverse polarity, or your LPF is set to 20hz.
Here is the entry out of
your manual that corresponds with hookup of two speakers (or in this case, two voice coils):
For your sub on your amp I would recommend wiring it like the bottom diagram. Treat each "speaker" in the manual page as ONE of your voice coils. It is wired INCORRECTLY in the photo you provided. Pay close attention to the polarity as well.
Just for testing set the LPF/HPF swith to "off" (the center position) for each channel.
Wire it like the bottom diagram, set both gains to approx 50%, LPF/HPF to OFF, switch at 2 channel (or 4 channel if you got the RCA Y splitters). Play Bass I Love You again, shoot a new video, and take a detailed photo of the settings panel on the top of the amp.
If the sub still doesn't move much except at that one frequency, try disconnecting wires from one voice coil, see if it preforms identically (on a short test).
Good luck.
-Jamie M.