I don't know about the warranty etc., but maybe you could try asking here:
Myapologies. The link I had given doesn't seem to work, so go to: http://line6.com/support and then I think "Contact Technical Support", then "Contact Technical Support" again I think Line 6 now use that for helping with problems.
I agree about asking about the knob. Better to ask and find out first before trying it with a lot of force and then breaking it
With my ux2, it looks like you could take the knob off ok. Not sure about the ux8.
Normally these things do just sit on, although sometimes a little screw can go through to tighten around the potentiometer's pole, but I don't think these ones do that.