The knob caps for the rotary faders are not push-fit but secured with a collet, washer and nut.
They are the same parts as used for the Wet/Dry Control knobs on some Xone:DB2 and Xone:DB4 mixers.

Care should be taken not to try and remove these Wet/Dry knob caps by simply pulling, this could result in damage to the potentiometer.
With excessive use, if your knob becomes loose or needs the alignment adjusted, removing the plastic cover at the top give you access to the nut.
(see attached photos)
If you need to order a replacement, please contact your dealer or local distributor

part no - description
AJ5613 - KNOB CAP BK-WH LINE 21MM      
AJ5612 - KNOB COLLET SOFT BK-WHITE LINE  (includes brass collet, washer and nut - see attached photo)

aj5612-knob_collet_soft_bk-white_line_aj5613_knob_cap_bk-wh_line_21mm

aj5612-knob_collet_soft_bk-white_line__aj5613_knob_cap_bk-wh_line_21mm__assembly