wondering if anyone knew which frequencies (Hz) the treble , bass are actually adjusting.

also semi - related, i am installing aftermarket subwoofer/amp and plan on tying the line level inputs for the amp on the wires that go to the factory sub, with the end goal being able to control the aftermarket woofer via the subwoofer level adjustment on the tlx factory head unit. has anyone else done this and had good results? does anyone know the frequency cut off / low pass filter that is sent via the factory head unit to the factory sub output?

thanks