The main advantage of using the expensive JBL driver could be a lower admitted crossover cut, e.g. at 800 Hz, as suggested in the 2426 datasheet. I haven’t read anything about 800 Hz cut in the forum. At least using the search function: I’m going back reading the topics but it is a time consuming activity, even if really interesting.
800 Hz seems to be a better cut for the couple JBL 2226-2426, but not allowed for any of the other driver mentioned.
I also found a schema for it in the documentation you gently sent me, Wayne.

Can I deduct this is an available option?
And in case, what would be sonically better for you, 1,6Khz or 0,8Khz?

A great advantage, in my opinion, would be that the 2226 has a lack in the off-axis response over 500 Hz which became “important” (-15db) at 1600Hz.

Off-axis response is not so critical in my listening environment, but I prefer to do things in the better (affordable) way.

To expound a little further, if you crossover a direct radiator to a 90º round horn, directivity is matched when wavelength is approximately equal to the diameter of the radiator. In terms of DI, a direct radiator has DI of 10 when diameter equals wavelength and DI of 16 when diameter equals two wavelengths. A 90x40 horn has DI of 11 and a 60x40 horn has DI of 12.5. That makes DI matching of a direct radiator to a 90x40 horn occur a little above 1 wavelength and a 60x40 horn about 1.5 wavelengths.

A 10" driver typically has a radiating surface about 7.5" diameter, 12" driver has about 9" diameter radiator and 15" driver has about 12" radiating diameter. So the frequency where wavelength equals diameter is 1765Hz for a 10" driver, 1500Hz for a 12" driver and 1130Hz for a 15" driver. That makes a 90x40 horn matched to a 10" driver between 2.0kHz and 2.4kHz, a 12" driver between 1.8kHz and 2.1kHz and a 15" driver between 1.3kHz and 1.6kHz. A 60x40 horn is matched to a 10" driver at 2.4kHz to 2.8kHz, a 12" driver at 2.1kHz to 2.4kHz and a 15" driver at 1.6kHz to 1.8kHz.

• More info about crossovers and driver spacing

The biggest doubt is, as I previously told, if using the JBL 2426 rather than the B&C DE250 or DE500.

I understood the crossover filter remain the same for both JBL 2426 and Eminence 2002, even if I’m not sure after reading some audioasylum tread.

Is it the same even for B&C drivers? B&C seem to keep (from official datasheet) a flatter response at higher octave, especially DE500, so probably they need a lower bypass capacitor for the attenuating resistance cell (C1 in your documentation), even if not at all. They seem to have the same level at crossover frequency.

You said somewhere you where testing B&C compression drivers some months ago. Have you done it? Do you have a spice model for them?

But, mainly, are they well sounding as JBL’s? And is it worthwhile the cost difference?

I know your point of view regarding JBL 2426 vs, Eminence 2002 but not compared with B&C.

Pleas let me a couple of days more to collect the last ideas and I’ll be ready to order you the components.

The last question: can you suggest me the right dimensions (diameter and depth) for a circular vent (or two) for the Professional IV (maintaining the same volume)?

Thank you

As for comparisons between the Eminence PSD2002 and the JBL 2426, there is more HF energy from the PSD2002 but the 2426 is smoother. They're definitely two different drivers with two different characteristics. That said, I still recommend using the same tweeter compensation circuit on my passive crossovers. The reason is that it provides the maximum augmentation possible from a passive network. With the Eminence driver, it's just enough to get flat average SPL up to 16kHz. The JBL 2426 will have falling response, even with passive compensation, but there's no more augmentation possible from a passive network. The augmentation is really just removing attenuation, so once all attenuation is removed, no more HF augmentation is possible.