To be honest, once you get to the X3ii, the differences up from there are minimal. I very much doubt that volume matched and given a choice of five daps with the same music cued to the same second, anyone would reliably tell one player from the next unless the DAC chips in one were skewed for warmth and another for accuracy. We passed the limits of practical improvement long ago.
But the differences in build quality, in design, in labour, in UI quality, in screen, in touch, in in-built memory, etc., are massive from the low and middle end on up.