in case if theres any confusion, the knob you have in mind is actually what most refer to as pot or potentiometer, the knob itself is just a plastic handle for easy control of the pot, which in turn affect the setting parameters.
Thus the problem you have encountered, is actually the pot giving the problem and not the knob.
Anyway, if the actual issue is with those pots, it can be replaced easily. If you are comfy with diy, those pots can be changed easily. If not, the shops which deal with modding and stuff should be able to check for you. For the shops, can do a search in the forums here. Theres always some names of the shop being mentioned, can call them up and check with them, soundalchemy, tymc, etc.