Please clarify WHERE you are trying to attenuate the audio signal. Are you trying to attenuate it at the INPUT to the power amplifier, or at the OUTPUT? It is not clear why the specs of the speaker are significant here if you are asking about attenuating the "line-level" audio signal going into the power amp.
Why wouldn't you simply use a potentiometer as a variable "volume control"? Depending on how you are deploying your gadget, you could put knob on the back of the console for adjusting the volume, or you could put it on the front where it is a user-control.
The larger capacitor will better pass low frequencies (along with the higher frequencies). When you say that it sounded "muddy" it seems possible that is more a function of the speaker and whatever enclosure you have it in. There are ways of calculating the value of the capacitor, but you wold need to know the "input impedance" of the next stage (the input of the power amplifier). It would help if you revealed what you are using for the power amplifier.