The Hsu amps have an equalization circuit in them that boosts peaking at about 19 hz and then cutting off at the low end. Essentially, the Hsu's are detuned a half octave lower than a true fourth order and then the boost brings them to flat down below 20 hz. The nice part of this is that in the range of boost, the port is radiating all the energy while the cone is not moving much (port resonance). Thus, the distortion remains very low
