I have all the divers for what you describe, but I expected I would need a much larger cabinet for such a 2.5-way. Is that correct?

But this gives me another idea:
Alpha 6A in its own sealed 9 liter chamber with NO crossover coponents to cover 200Hz to 5000Hz, DX25 as configured in Studio 2 for above 5000Hz, Delta 12LF in ported 100-120 liter chaber rolled off as you sugest for below 200Hz.

Sharing appartment walls with neighbors, my son hasn't the need for extreme efficiency and max SPL available from the horn-loadd systems

I'll probably try all these options eventually, but am curious as to what you would suggest first.

Thanks again.