Also keep in mind that the real bass rolloff varies with the impedance of your input.
In most cases, if you go line in - the bass rolloff won't do anything!If you go mic in - you have to know the impedance of the input to know what rolloff you're getting. Any battery box that is labelled with frequency rolloff
is making an assumption about the input impedance - if your recording device is the same, great. If not, the numbers are wrong!
Example: For the most (all?) of the Sony DATs, mic-in impedance is 4.7K, and 'line in' is 47K. A box with a rolloff at 120Hz on mic-in will only be 12Hz on line in (non-existent).
The same battery box plugged into a Sharp mini disc (10k mic-in, 20k line-in) will give you a rolloff of 56Hz on mic input and 28Hz on line-in.
Also - the rolloff is a slope, not a sharp cut so it's not that you get 0 below the cutoff - that's just where the slope begins. The steepness of the slope can vary but most boxes will be down 6dB per octave. That's -6dB each time you halve the frequency. So for the same example above on a Sony DAT using mic-in
120Hz - no effect
60Hz - down 6dB
30Hz - down 12dB
You can get a feel for how much that is by trying it with software on one of your recordings that wasn't rolled off.
If anyone wants a relatively easy way to do these conversion between devices just let me know and I'll post it. I know I tend to get excessively technical sometimes....