The bump filter is a 12 dB/octave high pass filter with a high Q to give a 3 to 6 dB rise in response at the tuning frequency of the box. It's mostly port action at that frequency, the driver is doing next to nothing. Below the box tuning frequency the system response is falling off at 30 to 36 dB per octave due to the bump filter rather than the usual 18 dB to 24 dB per octave of a typical vented box system without EQ. This helps protect the driver which no longer has any benefit of restoring force from the box (it has unloaded) but it also mitigates any benefit of room gain, which usually occurs as a 12 dB per octave rise (that's why little sealed boxes such as the B212 and TiK sub go so bloody low).
Remember that the 4645 has a specific target specification that has to balance performance with longevity under duress. Yes, there are cheap little subs that will wipe the floor with the 4645 between DC and 20 Hz in a home but they will never, ever fill an entire auditorium with 25 Hz information with any kind of authority. That's why we often use home hi-fi subs instead. Technically the 1500 Array shouldn't be any better since it also uses a 25 Hz bump filter. However, it has a little bit more area under the curve than the 2242H does so it seems like it goes lower. A 2245H instead of the 2242H might give you more of what you want. Both drivers can also be tuned down to 20 Hz and the corresponding 20 Hz bump filter applied. Power handling will be reduced. It was never a problem for me personally in my home.