Potentiometer/attenuator between amp and speaker
Let me try and put you off using a potentiometer at the output in series with the speaker. Let's say (for argument) that you'd like to control the speaker level to about one-tenth of normal full-level. That would require adding 80 ohms in series with an 8 ohm speaker thus the voltage level on the speaker is about 10%. OK so far. You would use an 80 ohm pot (more accurately a rheostat).

Now if you turned the pot up so that there is the same voltage across both speaker and pot each would dissipate equally the same power. If the speaker power is 50 watts then the pot would also dissipate 50 watts but, it would dissipate it on about one-tenth of the pot track.

So, the pot you would need would have to be one that is rated at 500 watts. It's going to be the size of a small dinner plate and cost beyond $200. In other words, you are probably better off just buying another amplifier and using its volume control to give independent channel amplification.