Thanks for your response Gs. I now see that many power amps do typically offer high balanced input impedances.
What led me astray are these lines from the Threshold S/350e owner's manual: "The balanced setting provides a standard 600 Ohm professional connection, and the unbalanced setting provides 50,000 Ohms load through an RCA connector." and later, "Moving the switch toward the XLR connector will select a 600 Ohm input characteristic. Moving the switch toward the RCA connector will select a 50,000 Ohm unbalanced input characteristic. Make the appropriate selection as required by the equipment sourcing your Threshold amplifier." and finally in the spec sheet, "Input impedance: 50,000 Ohms unbalanced, 600 Ohms balanced." Maybe my question should be: what is the standard 600 Ohm professional connection?
What led me astray are these lines from the Threshold S/350e owner's manual: "The balanced setting provides a standard 600 Ohm professional connection, and the unbalanced setting provides 50,000 Ohms load through an RCA connector." and later, "Moving the switch toward the XLR connector will select a 600 Ohm input characteristic. Moving the switch toward the RCA connector will select a 50,000 Ohm unbalanced input characteristic. Make the appropriate selection as required by the equipment sourcing your Threshold amplifier." and finally in the spec sheet, "Input impedance: 50,000 Ohms unbalanced, 600 Ohms balanced." Maybe my question should be: what is the standard 600 Ohm professional connection?