Like I said, you need to do this scientifically. You need to find out the impedance of the speaker circuit. The speaker may be marked, or you can ask MTH. Then you need to use a potentiometer, wired as such, (three connections) not wired with two connections...that's a rheostat...
Using a potentiometer that has a high impedance wired as a rheostat, will not significantly reduce the speaker signal, and you will probably not even notice any reduction in volume.
You need to create a voltage divider circuit---that's what a pot does...and you will then have a linear volume control that will allow you to do an infinite adjustment.
Once you know the impedance of the speaker, we can continue and you'll be satisfied.