40 Hz is a very, very low audio frequency, and a filter set to that frequency won't affect much musical material because there is hardly any musical material down there to begin with. Also, pressure gradient microphones (any kind of directional microphone) have considerably reduced acoustical pickup at or below that frequency, but they have considerably increased sensitivity to wind, breath noise (for close-miked vocals or speech pickup) and mechanical vibration/shock noise, so that by the time you're down around 30-40 Hz, the majority of the signal energy coming out from them is garbage.
Pressure transducers (which are omnidirectional) are a different kettle of fish, but live music (especially pop/rock/jazz) doesn't often go that low, and field recorders such as the SDs are most often driven by directional microphones. So the main purpose of a 40-Hz filter on a recorder like that is for safety--to keep low-frequency noise from overwhelming a recording.
It's not a tone control in most cases; it's too low for that purpose, and it's the wrong shape; it's a filter, whereas most tone controls that are designed to improve musical balance have a "shelving" response. In other words there is a region which they don't affect, and a region which they affect by the amount you've dialed in, and of course a transition region in between--but they don't continue cutting or boosting further once you reach the frequency at which they take their full effect.
So don't use filters as tone controls; they generally won't do the job you intend. Use filters as filters, and tone controls as tone controls, is my suggestion.
--best regards