Volume reducing resistor in speaker lines
Adding resistors in series with the speakers as you suggest would work, but is not a good idea, for two reasons:


Additional distortion: the impedance of the speaker is not exactly 4Ω over the entire range of frequencies, whereas the resistor will have a constant resistance, whatever the frequency. So it means that, depending on the frequency, the speaker will see a different RMS voltage across it (the frequency response will be less homogeneous).
Wasted power: dropping the level to 1/8th means that the resistors will dissipate most of the power from the amp, and this is just wasted as heat (and you'll also need bigger, more expensive resistors).


It would make more sense to attenuate the input to the amp, using another pot, or just a fixed resistor divider (actually two, one per channel).

Here is what you can do, for each channel:







simulate this circuit &ndash; Schematic created using CircuitLab

Adjust R2 to get the level you need (the lower the value, the lower the volume). Here, with a 4.7k resistor, you get about 1/3 the voltage, so it means 1/9 the power, and is pereceived as about half the loudness. If you put a 1.1k, you'll get 1/10 the voltage, so 1/100 the power, and it will be percieved as about 1/4 the loudness.