Look at the bentaudio website [www.bentaudio.com] John has a complete motorized stepped attenuator kit available for $200.You will also have to choose from either Gold point,dact or Shallco switches [any of these will provide better performance than a pot].For $300 you can get the assembly complete with a shallco which would be my choice for your unit.I am sure John will help you choose the right value and type.
