Attenuator for guitar amp cheap DIY
Two power resistors can do this. For example, 8 &Omega; across the speaker and another 4 &Omega; in series with both would put only 1/4 of the output power into the speaker while still presenting a 8 &Omega; load to the amp.

Consider the power requirements of the resistors.  In the example above, only 1/4 of the output power will go into the speaker, so 3/4 will heat up the resistors.  Half the total power will be dissipated by the 4 &Omega; resistor, 1/4 in the 8 &Omega; resistor, and 1/4 in the speaker.  If the amp dumps 40 watts, for example, then you need 20 W and 10 W resistors minimum, respsectively.  In practise you want to get resistors rated for somewhat more than the maximum power you plan to have them dissipate.  Those big ceramic wire wound types are probably what you will end up with.