I have confused myself again.

I am using 2446H drivers and would like to experiment with different diaphragms. The Truextent Be diaphragms are too expensive ($950 each).

However, some you have suggested using an aquaplas coated diaphragm for a smoother response (although the high frequency extension may be a bit reduced. The impedance is not an issue since I am using DSP crossovers.

According to Simply Speakers they have genuine JBL diaphragms (coated with aquaplas) for either the 2450 drivers or the 2451 drivers that will fit a 2446 driver.

The 2450 driver uses a 4 inch voice coil with 1.5 inch exit (cost of the SL diaphragm is about $80 and they only have the 8 Ohm) Part # JBL D8R2450SL

The 2451 driver also uses a 4 inch voice coil but the driver itself is a 2 inch exit (like my 2446) and the cost is about $125 and they only have the 16 Ohm version (and not currently available).
Part # is JBL D16R2451SL

Again, I am not worried about the impedance, so what am I not understanding? Although both diaphragms are designed for a 4 inch VC, I guess the design for a 1.5 inch vs a 2 inch exit or the phase plug must be the difference? Which is the better choice? BTW, both are smooth (not ribbed).

Thanks,
-Tom