Just pull it straight out. The knob just holds by a metal spring that clamps around the shaft of the potentiometer/switch combo. Like the usual potentiometer knobs. Mine was a bit rusty and also seemed to be secured by the reddish loctite-like material that is on every screw.
Took a LOT of effort though. Much, much more than I had expected. So I was terrified to break anything, too. But in the end it came loose with a bang and everything was ok. Don't lever on it, just try to get a good grip and pull. Be prepared for the sudden release.
PS: the reddish loctite-esque stuff can be cleaned from the screws and heatsink and such by submerging the parts in a water/simple-green-substitute solution. Actually I used Koch Chemie Green Star, but should be the same. Pure water seems to work, too, but takes a lot longer (days instead of hours). The "loctite" gets foamy and can be simply brushed away after a night in the solution.