Thanks for the help. Looks like for this amp, I should go with a 2 ohm for Resistor 3, and 6 ohms for Resistor 2. This reaches the effective speaker load of 7.9 ohms which is nearly 8 ohms.
This creates an attenuation of -12.37 dB, which I think should be sufficient.
Can this even be done on a single cable or would you need some type of enclosure?
If you are careful with the connections, sure you can do it in the cable. Just make sure everything is insulated with heatshrink so you don't end up with a direct short. Also, upsize the resistor power dissipation rating, since everything would be insulated and won't have the chance to dissipate heat as if it were in free air.