* "Basically" = as in, the Sound Pro battery box is set for an input impedence on the recorder of 10 kOhm, if I recall correctly. Obviously, the actual rolloff value would differ per recorder with the input impedence being a variable in the following bass rolloff equation: 1 / (2*pi*R*C)
where pi = ~3.14, R = Farad of the capacitor, and C = Ohm of the recorder's input.
Correct-they are calibrated for 10 Kohms (old mini disc mic in). So you will get way different results with recorders used today. I never heard about the equation mentioned by BlingFree (and am not knowledgeable enough to use it), but it sounds like he knows what he's talking about. (The old posts I read said that if you use a recorder with input impedance of 20 instead of 10, for example, you would get 10/20 of the roll off listed by Sound Pro's).
The Sony M10, for example, has mic input impedance of 22 and line in impedance of 40 (way different than 10). Who knows what roll off you'd get.
I don't think SP's should be offering these boxes for the above reason-too confusing to operate (and they don't tell you about the impedance issue). Most of us here recommend doing all roll off in post anyway. Chris Church's preamp's HPF, which normally should not be engaged except in extreme circumstances, operates differently. If engaged you will get the same bass roll off no matter what the recorder's input impudence is. Unlike the SP battery box, the roll off is not adjustable, but sometimes simplicity can be a good thing.
If you read a lot of posts here, most of them would probably have recommended buying a battery box without a roll off, I think. Since you have paid the extra money for this box however, it will work without issue if you just set it to minimum roll off and do your roll off in post as usually recommended here.