On flip the polarity, he means to swap the wires on one of the subs. If they're in the same box with no divider, one would be canceling out the other.

By the schematic, he means an electrical diagram for the remote input and for the knob itself. If you get that, any knob could be made to work with that amp.

What kind of amp is it? And did you check ebay for a knob?