Keta has hit the issue firmly on the head: the 970 is a pure processor, so the only component that directly sees the speakers is the Model 7100 amp. The amp does not include a selector for ohms because it is equally stable with 4 ohm speakers and 8 ohm speakers. Receivers that rely on the 8/4 ohm switch typically use it reduce the rail voltage in the 4 ohm position to prevent overheating when low impedance speakers incur a greater current flow. Separate amps lack the switch because they simply serve up the extra current.