[shovelhd]

Pug,

I don't know what "SCTY" stands for. Nothing comes to mind.

I don't know if the sub output is conditioned, i.e. equalized, limited, crossed over, etc. In general, you can use any speaker level signal to tap a sub amp input as long as the amp has high level inputs and a crossover built in. Many sub amps have two (L+R) high level inputs so you can tap both sides. You will lose the ability to control the sub level from the HU.

What you're seeing with the volume mismatches is an impedance mismatch. The aux input is designed for a certain impedance signal. If the impedance is exactly right, the levels between the two devices will match, and all is well. When one is off, it can be too soft or too loud (overdriven).

Differential inputs are used when grounding one side of the signal is not desired, i.e. in high noise environments. There are two signal wires and no signal ground (although there is usually a shield and one end is grounded). The amplitude of the signal is the difference between the signals on the wires, instead of a voltage above ground. I don't know what the OEM HU uses on the aux input.