Bonjour Stéphane,,,,

They are physically aligned +/- 2 cm, the xover are :

PLLXO at 270hz
Bi-amplified
passive low pass 1200hz, then band pass 1200hz > 5800hz the high pass 5800hz.
All 6db 1st order.

The bass is inherently weak below 50hz (vot) but the upper bass are so musical. Need to fill below 70hz with something.

What about a tapped horn?

Have some 416-8a or 515b's spare so maybe they can be used for a sub???

Tim