EW, the problem here in part is terminology. Subs have no internal "crossover" although manufacturers sometimes(mis)label a control on the back as such. A full crossover can be done in the receiver, which rolls off the sub above the selected frequency and the connected speakers below that frequency(typically at 24dB/octave). The component and control in the sub is simply a low pass filter(LPF, as the Axiom graph correctly labels it). This rolls off the sub above the selected frequency, but can of course have no effect on the speakers.
Examining the EP600v4 frequency graph, it appears that there is a very sharp digital LPF, with a roll-off approaching 40dB/octave above the selected frequencies. On the low end, a sealed enclosure typically "naturally" rolls off at 12dB/octave below the enclosure tuned frequency(apparently about 20Hz for the EP600 enclosure). This possibly is the context for your 12dB mention. The very sharp low end roll-off shown in the graph is the result of switching in a 20Hz subsonic filter, which generally isn't used.
So, the bottom line if you have a receiver with crossover setting provisions is to set that at the appropriate number(e.g., 60 or 80Hz)and set the EP600v4 LPF at the highest 150Hz setting to get it out of the way of the crossover that the receiver circuitry is doing.
