Yes, a user supplied external amplifier is required. Amplifier level signals are measured with a pass through port that measures the voltage and current. The port itself can handle up to 40 Vrms and 5 Arms (a 200W 8 ohm class of amplifier). If you think about it, this is actually a pretty high level given that the high power TS tests are being done in open air and at Fs. One restriction is that the amplifier needs to be ground referenced, so you cant use a bridged amplifier. This is usually not a problem for amplifiers in this size range.
Another suggestion is that the amplifier should have a manual volume control or be used with a preamp that has a volume control. Playing an unwanted WAV signal from an unintended audio device, and at maximum level can be a devastating and undesirable experience. A simple *manual* volume control will put a limit on how much output you can get.