By reactance annulling you can gain extra bass near cuttoff by aligning the reactive part of the drivers "load" below resonance with the air mass reactance in the throat of the horn. May sound more interesting if it were called "free bass"(!)

Regards Philip

The simple explanation is that a horn is most efficient to Fc if the system resonance is close to the Fc. When system resonance is below Fc you push it up higher by making the compression chamber smaller. This ties in with why you don't want to start with a driver Fs below Fc, because the acoustic impedance load of the horn and the front chamber push the effective Fs down, and can make the size chamber required to get system resonance back up near Fc impractical or even impossible.

Like Phillip said, essentially set the sealed rear chamber to shift fs to match fc. The rear chamber is small so this results in an underdamped sealed system with Q much higher than 1.0. You can see this with T/S programs. Where the horn response falls, the sealed chamber response peaks.

In effect this "mitigates" reactance (I like your choice of words) at fc because horn impedance is imaginery at that point. Impedance swings alternating resistive and reactive for an octave or more above cutoff. The amount and severity of impedance peaks are set by horn characteristics. It is most severe on undersized horns. In any case, impedance becomes more and more pure resistance as frequency rises.

The peaked rear chamber makes the horns first reactive peak act more resistive if done properly. It cannot mitigate every peak but it can address the first and largest one. It fills the response dip which also means it has made impedance more resistive there. Any system at resonance has counteracting reactive properties, making impedance resistive.