@amirm It is definitely the time to be testing the protection mechanisms integrated into these products (hopefully) to save attached (and expensive) headphones from accidents.
I suggest the following:
1) DC protection.
Inject an offset of x mV and wind up the volume. Monitor the output and call the level at which the amplifier triggers its onboard DC trip (or not).
2) Short circuit protection.
Wind up the level to full rated power and short the outputs. Describe the protection (or lack thereof) behaviour.
3) Overdrive/Overpower protection
Is there a level where the device enables protection based on clipping (running into the rails) or excess current consumption?
Traditional amplifiers have had these protection modes (and others) for many decades and now headphone amplifiers are getting serious in the power department, these parameters should be tested and/or commented upon.