Butterworth filters
Pro-Q 3 does Butterworth high pass at default. ReaEQ can do the same with a 1.90 Q factor. Learn more about Butterworth filters here and here.
Notes mentioning this note
All about eq and filters
All about EQ and filters [[Parts of the frequency spectrum]] [[High-pass filters]] [[Use vowel and consonant sounds to help you...
Have no fear about high Pass filtering
Have no fear about high-pass filtering
I did some investigation after watching this Paul Frindle video.