Hi Ivica,
The impedance of the “naked” D2 does indeed shift quite a lot over the range. The passive filter does really matter as can be seen in the plots below.
All is D2/2430K marked “M2” (brand new and I measured two different samples) on a new M2 wave guide, no baffle.
First plot is swept impedance taken with DATS II (calibrated)
Second plot is with the JBL M2 passive filter
Third plot is frequency with the correct BSS DSP correction and JBL M2 passive filter
Not too shabby! Please bear in mind that there is no smoothing at all applied here.
Best regards