General setting for biamp 2x15”+2”

2) Measure on axis frequency and distortion response of 2" section

3) Because you are looking for roughly 600-1200Hz crossover based on past knowledge with similar components, measure horizontal off axis frequency responses of both sections and look within this octave for frequency range where they share the dispersion, ie have similar -6dB point compared to each section's on axis response.

4) To make vertical dispersion as good/even/without anomalies as it could, you should aim for as low crossover as possible but you need to make it a compromise between sections having equal horizontal dispersion over overlapping band and each section's distortion behaviour. Generally it is wise to cross over high enough before compression driver starts to distort trying to reach lower frequencies. This would generally somewhat correlate with manufacturer's recommendeded lowest crossover.

5) There are many other variables and things you could do. Should you correct both section's on axis responses to be as flat as they can around the bandwidth they are going to handle before doing everything else? Should you try to have gap or ovelap in the electronic crossover to make things better in acoustic realm? How do you compensate possible level differences between 2" and 15"? Just by adjusting the gain or maybe having some wide band shelf filter on 2" for example to tilt the basic tonality in some way? How about FIR filters if you have the option?

TLDR; Crossover is decided with knowledge of distortion, amplitude and dispersion of each component.

Your best bet really is to try to find someone you could bribe with a few beers who has experience of setting up systems, who ideally has some basic measurement kit.  Once someone has demonstrated some of the things you can do to tame them, you could probably do some tweaking yourself but it does sound like you might need some help on the ground just to get you headed in the right direction.

Without knowing the cabinet tuning you'll probably be safe with a high pass as low as 50-60Hz.  You can probably also put a bit of gentle gain in the 50-100Hz region to improve the low end a bit.

Just working off the Delta15LFA specs, try adding an EQ point to the 15" section on the crossover, centred about 1.8kHz, Q of about 1.5, -6dB.

You may also need to go below -10dB HF as the 15's are not very efficient.  As I mentioned on a previous post, as a very rough starting point, try adding a 6dB/oct high shelf to the horn, about 6-8kHz and about +6dB.  That should start to bring a bit of the very top end back.

What you're probably hearing at the moment is that both the LF and HF may have large lumps in their responses around the 1-2kHz region.  You could also try doing a non-symmetrical crossover where you actually introduce a gap.  e.g. low pass the 15" at 800Hz and high pass the horn at 1200Hz.  Those numbers might be well out but gives you some ideas of things to try.