 Solution

Use the decorator pattern. The wrapper implements the same object interface as its component and delegates before changing the output:

class ScaledChannel(inner: Channel, factor: Int) extends Channel {
  override def apply(n: Int): Int = factor * inner(n)
}

It can wrap any channel software object, not only a sum channel. Constructing the wrapper at run time chooses the additional behavior for an existing component, and several wrappers can be composed without adding a subclass for every combination. Each call reaches the wrapped channel once, preserving its state evolution. Scaling the input instead would define a different transformation, particularly for a component that already has history.