 Solution

Override the operation and scale the result of exactly one superclass call:

class ScaledSumChannel(factor: Int) extends SumChannel {
  override def apply(n: Int): Int = factor * super.apply(n)
}

The superclass updates its previous-input state once and returns its ordinary sum; only that returned value is scaled. The inputs and superclass history are unchanged by the scaling. Reimplementing the state update is unnecessary and could accidentally diverge from the superclass contract. As in the supplied channel, arithmetic uses the declared fixed-width integer type.