It's more complicated than you think. The secondary winding is in the form of two tapped windings, which are used in various combinations of series or parallel. This is to avoid the problem of best performance on the 16 ohm tap with deteriorating highs on lower- impedance settings.
You would have to make a switch circuit, using two DPDT switches or one 4-pole 4-throw switch per channel. At one time, we had a circuit board that mounted on tall standoffs just below the OPT transformer, for the OPT-2. But between slow sales, rapidly rising switch prices, and the clumsy of disconnectingpower and signal lines in order to turn the heavy amp upside down to access the switches, it was discontinued. In hindsight, an external box would probably be more practical.
