How to link this

[d.healey]

if (link.getValue())
    presenceKnb.setValue(value);
    bassKnb.changed();

There are two things wrong with this.

1. You want to udpate the presenceKnb not the bassKnb, you're in the bassKnb callback already.
2. You haven't put the command in the if statement - https://softwareengineering.stackexchange.com/questions/16528/single-statement-if-block-braces-or-no