You could possibly ask the developer to add high-pass and low-pass filter options. It wouldn't be too difficult.

As Math Audio's Headphone EQ has the ability to display the frequency response curve of the EQ settings, you could give things a try as follows.

For the low-shelf filter to behave a bit like a high-pass filter, at least over a limited frequency range, you would need to apply a gain of say –12 dB, place the frequency at 20 Hz, and set the Q equal to 0.7071. This produces a response that is –6 dB at 20 Hz, and about –3 dB at 28 Hz or so.

Increasing the frequency setting to 30 Hz produces a response that is –6 dB at 30 Hz, and about –3dB at 42 Hz or so. See below for what the frequency response looks like:

View attachment 249429
Is the above representative of the type of filtering that you wish to include? Increasing the Q to 0.8 will also make the transition band a little steeper. Higher Q values than that seem to introduce a noticeable amount of peaking in the response of the low-shelf filter.

If you cascaded 2 low-shelf filters together, you would of course get a steeper slope after the upper corner frequency.