I fixed my knob ages ago by wrapping a layer of black electrical tape around the metal knob. It has never slipped and the knob slides on and off easily!! I was doing it as a temporary fix before I got ahold of support, but it has worked for me this long (years)...
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Ryan here... Empeg [08000462] 40 Gig with Subaru WRX sport wagon attached... (still! pending memory + LED upgrade, sheesh, I've been sitting on my ass for years)