In older tube and solid-state gear, the 600-ohm output transformer was designed to terminate into a 600-ohm load. When some vintage preamps were removed from their original consoles and wired into more modern systems, which nearly always had high input impedances (10K-ohm and above), the result was a very slight rise in high-frequency response. Because some engineers prefer that sound, we made the output transformer 600-ohm load resistor easy to switch in or out of the circuit.