When I mix with my Xone:4D, the cross-fader does not respond as I expect: moving from X to Y the smallest movement seems to bring in the other channel very abruptly.
Check how the XFADE curve control on the top panel of the mixer is set.
This sounds like it may be set, fully clockwise, to 'cut', or closer to this extreme than to the ‘smooth' (><) setting.
If you want a smooth transition between channels set the curve fully anti-clockwise ><.
If you have assigned the cross-fader as a MIDI controller for your software application for control of a 'virtual' crossfader, the XFADE curve control on the mixer top panel must be set, fully anti-clockwise >< for the software to respond correctly.
It is not possible to use the hardware crossfader set to 'cut' and for the control to be used as a MIDI control simultaneously.
If used as a MIDI controller, check how the 'virtual' crossfader curve is set in your software application's options for the parameter, as this may be set to Smooth, Sharp, or any point between the two.