One side to the 8V audio output, one side to the signal ground, and the
wiper to the low voltage amp input. Use a pot in the area of 1K to 4K.

dB = 10ln(A/B) = 10ln(20/21) = -0.4dB

Sorry. Won't be effective.

The 200K series-only pot idea doesn't work because the impedance will
become too high. The impedance to the amp should be no higher than a
few K Ohms to avoid loss of trebble, electrical noise pickup, and
distortion due to non-linearities of the amp input's impedance.

What nonlinearities?

Um...and he can't set the amp's input impedance. It's predetermined by the
10kohm or greater resistor inside the chassis. He'll modify the input
impedance some by providing a series resistance, but it still shouldn't be
too high unless he has to introduce some major attenuation.