You could email the US distributor for VivA (firstname.lastname@example.org). They are extremely helpful.
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The "controller," which you hold in your hands costs nothing. I would want a small manufacturer to select a reliable and cheap model and not waste time and money reinventing the wheel. The cost comes from the motorized volume pot or resistor ladder/relay or whatever implementation of the attenuator inside the unit. I know of a manufacturer who has only recently found a suitable remotely controlled attenuator, having dismissed most of the motorized pots as not usable in his particular design. It is the attenuator that matters, not the thing you hold in your hand.