Clipping and clipping recovery behavior.
I've seen way too many power amplifier that either clip ugly (with sticking) or even burst in oscillation when clipping. The common design solution is the ubiquitous Baker clamp around the VAS gain stage, which is in way too many cases just a poor band aid. A true clipping protection requires a control loop that kills the amplifier loop gain when approaching the clipping levels. Not trivial to design, and not trivial to implement either... As any control loop it requires its own compensation to avoid stability troubles.
IMO, the clipping behavior would be one of the easy ways to discern amplifiers in a DBX test.