You can make an RC filter which will drop-off at 6dB per octave.
Here is an online calculator.
Note that with ANY filter the cutoff frequency is
defined as the -3dB point so if you calculate/design a "100Hz" high-pass filter it's 3db down at 100Hz. Then at -6dB per octave you'd be -9dB down at 50Hz. If you want to "fully pass" 100Hz you'd have to calculate lower cutoff frequency.
It can get a little tricky because in a high-pass filter the load (i.e. the power amp input) is in parallel with the filter's resistor, lowering the effective resistance and that increases the cutoff frequency. If you know the amplifier's input impedance you can calculate the effective resistance, otherwise the resistor should be low relative to the amplifier's input impedance. But, if the resistance is too low it will "load" the preamp output.
So usually, 10K is a good compromise assuming the amplifier's input impedance is about 100K.
...That's one of the "problems" with passive filters if you don't have control of the associated circuitry. Plus RC filters have that slight-slope and more complex LC filters can be lossy.
But, it may be "better than nothing".