Excuse me, but how is he going to do that? Pardon my lack of knowledge. Can you elaborate a little bit more about the shelf thing?
Cutting everything below 300 Hz would look something like the 1st pic attached.
A sloped rolloff looks something like the 2nd pic attached. (Not sure of the proper term, but when I visualize a shelf, I think of the first image; slope, I visualize the 2nd image.) At any rate, the idea is to reduce volume gradually across a frequency range, rather than all at once.
Most in-line high pass filters I've seen (usually on preamps) reduce levels by 6 dB or 12 dB per
octave. Every halving / doubling of a frequency = one octave down / up respectively. So if we start at 300 Hz and apply a HPF of -6 dB / octave, our slope is 0 dB at 300 Hz, -6 dB down at 150 Hz, another -6 dB down at 75 Hz (total of -12 dB relative to our starting point at 300 Hz), and still another -6 dB down at 37.5 Hz (total of -18 dB relative to 300 Hz). A -12 dB / octave slope would be -12 dB per each octave, or 0 dB at 300 Hz, -12 dB (total) at 150 Hz, -24 dB (total) at 75 Hz, -36 dB (total) at 37.5 Hz. I typically find a -12 dB / octave slope too severe and usually start with a -6 dB / octave slope as my starting point.