I am not familiar with your driver and I do not know those horns. It looks like they are 200Hz horns through….
You shall not worry about your horns create any shadow. As I understand you use digital crossovers with your horns and if so then you can potion then in any more convenient anti- shadow fashion and then alone them with time-delay within digital domain. Well, it is if you feel a need for time alignment…
The DVB15N horn does not go down to 60Hz as it said in this specification. It is basically 200 Hz -250Hz horn and the lower then this will be pushed by direct radiator and will have no gain from horns. I generally do not like those horns and I feel that people shall be better off with conventional properly made direct radiators. There is a number of threes at this site about the problem of upperbass horns design and implementation, you might find it worth to read them.
Rgs, the Cat
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche