10W Class AB - Almost a Success, then the Smoke Escapes

I tried the first alternative transistor and saw mixed results. The distortion on peaks was still present. I slipped with the attenuator control and pushed the amp to 20W. Briefly. Then all the emitter resistors let go. 

The distortion has me puzzled and is the same as the first amplifier with 10 parallel transistors. So I Googled and saw this waveform:

 

 

The term "sharkfin" describes it well. The distortion in my amplifiers can be similar when pushed hard which does point to harmonics being present. The amount of distortion at 10W appears to depend on transistor type from results to date.

 

It appears I'm close to achieving 10W of output with 10 parallel SOT89 transistors. I'll replace the resistors and give it another try. A post with more detailed test results in due course.