This improved things a little, but not by much, so I loaded up Axidriver and started to simulate what flange shape should provide the best response. Simulation proved fruitful, and small gains were achieved, but nothing outstanding. The widening of dispersion was still a problem as were the on-axis issues. I wasn't too concerned with the on-axis response but I did want to try and tackle the widening of dispersion. Axidriver indicated that this could be down to the cone geometry, and if that was true, there wasn't a lot I could do about it anyway. The SB17s cone isn't straight sided it's made up of two profiles. A steeper one near the voice coil and a shallower one near the surround with a smooth transition between the two. Altering the geometry in simulation had quite significant effects on the way the cone affected the overall dispersion of the tweeter.
For inspiration I started to read Brandon's (augerpro) waveguide thread over on the Parts Express forums. When his first set of measurements, for the elliptical waveguide, showed up that helped explain something. First of all the horizontal response was excellent but the vertical showed the exact same dispersion widening that I was seeing. To me this showed that you could have a very good throat interface with the tweeter, as shown by the perfect horizontal response, but if the waveguide geometry wasn't right, as with the compromised vertical geometry, you'd get a less than perfect response.
Well that was it and I thought I was done. I'd have to accept that there was an inherent limitation as imposed by the SB17s cone but then I read the rest of the thread. Right near the end Brandon summarised his opinions on soft domes Vs metal domes. Basically he said that soft domes appear to only perform decently in oblate spheroid shaped waveguides whereas metal domes work much better in other shapes. Well the SB17 was most certainly not an oblate spheroid, the mouth looks to be conical, so I figured it would be worth trying out a metal dome. Thankfully I had some Aura NT1 tweeters left lying around, one of which was my first experiments for use with the FST. This tweeter was rather beaten up and the dome had been poked by mistake a couple of times but it still worked. So I came up with a new 3D printed flange and tried it out.