The QA450 is a switchable 4 or 8 ohm programmable load for testing audio power amplifiers. The QA450 isn't designed for sinking power from your amp for extended periods. Instead, the QA450 is designed for rapid testing using burst stimulus as generated by the QA401 or any source that generates a burst or chirp.
Below is a video of the QA401 and Q450 working together to test a class D amplifier. For certain tests (efficiency) you'll see the power peak around 150W into the QA450.
The QA450 is thermally protected, meaning it will self protect and disconnect the load if the internal load resistors exceed a safe thermal limit. The peak thermal load the QA450 can absorb is 200W at 8 ohms (300W at 4 ohms). The average thermal load the QA450 can withstand is around 4-5 watts per channel. So, if you subject the QA450 to 300W at 4 ohms for 400 mS (equivalent to a 16K FFT at 48Ksps), followed by a range of other tests at at much lower power levels, your average power over a 25 second test interval will hit the 5W target. See the manual (linked below) for a discussion on thermal considerations and some calculations.
The QA450 can also sense DC current (with roughly +/-30 mA of accuracy, up to 15A), and provides a soft-start through a 10 ohm resistor so that you can gently turn your your DUT. This allows you to use a low-cost power supply instead of a much more costly lab-power supply.