The resistor should not be in series with the OT secondary and the speaker connection. The resistor should connect from the 8 ohm tap to the cathode of V2-A. Alternatively, you can connect the resistor to a switch at the cathode of V2-A. With a single pole, three throw switch you could have negative feedback applied, no negative feedback with non by-passed cathode resistor, or no negative feedback with by-passed cathode resistor. Coming from the 8 ohm tap, the value of the resistor should be 56 K.