Let's say you want to attenuate the power by 50% at your lowest given freq of 80Hz. If the amp's output resistance is 4Ohm, adding another 4Ohm impedance in series at 80Hz will accomplish this. Frequencies below 80Hz will be attenuated even more and above 80Hz less

So Capacitive reactance(Xc) = 1/(2*pi*f*C), where f=Hz, C=Farad

C works out to be about 497uF

Because you need a non-polarized capacitor for audio (which is AC) and non-polarized caps of this value will be expensive if you can get it, you may use 2 electrolytic capacitors each of 1000uF connected in series (back to back), positives to positives or neg's to neg's to create a non-polarized cap. Voltage rating of each cap should be about 25v each. This will be available from your local RadioShack or electronic parts store.

Hope it helps