For home use, the HF108 has more than enough output if crossed around 1kHz on a conical horn.For the current use, the HF108s would be great to below 1khz. However, on reflection it would be prudent to allow for future eventualities, needing more SPL - such as for Home Theater use, or bigger rooms than the current one. That could mean crossing higher (to smaller/more woofers), but I think there are still other reasons why crossing low remains desirable.
In that latter case, a compression driver with bigger diaphragm could make sense.
The purple trace in Mark100's Syn11 is the response of four B&C NDF34, nicely covering 180Hz to 1kHz.
On a conical horn, I'd prefer their sound in the 400-1kHz range over any compression driver.
Mark is crossing them ~700 Hz to the B&C DCX 464 coax compression driver.
Four NDF34 can do ~116dB at 160Hz at one meter within Xmax, they can be crossed to low frequency drivers mounted internally or externally.
I've listened to them outdoors on a horn with a mouth the size of the back chamber in the above photo playing above three EV MTL4 (quad 18") subs, they got louder than I'd care to listen to at under 8 meters or so.
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