Hit up your local Kicker dealer. Bass knobs are usually rare to find on an audio forum. This is because either A) Someone is already using it in their set up or B) They are selling this with their amps
I have an Adire REVO bass knob. If its the phone cord style, Id assume they are interchangeable. Its plain black, in case you wanted to put the kicker logo on it or whatever.
Polecat will have to chime in, but I think the only bass knobs that work with the SX amps are the SXRC's. I don't think they have a standard bass knob. I know they use a CAT5 cable for the remotes.