No not maximum attenuation. -12dB per Octave. Every time you drop an octave (half the frequency) you're another 12dB down. Take the example of a -12dB/Octave HPF with a cutoff frequency of 200Hz (obviously high for bass guitar use, but just for illustration). At 100Hz the filter is cutting 12dB, at 50Hz it's cutting 24dB, at 25Hz it's cutting 36dB, and so on.