Do you use Roon, or have some other DSP processing capability? You could add a bit of a boost around the crossover point to see if that helps.
Also, if you haven't done so already, play around with the phase control to see if you get any improvement. You could be getting some cancellation around the crossover point although I think it's more likely that the single NEO10 is just running out of steam at the bottom of its range.