The BD139 Thing is a 300mW RMS stereo Hi-Fi Class-A amplifier.
It is pairable with a set of very high sensitivity (>90dB) speakers for audio playback.
Distortion analysis has not been done yet, but THD/IMD is probably 5-10%; most of it is pleasing "tube sound like" distortion, transistorized.
I am working on another revision of the design. The BD139 cascode is modular, meaning it can have many in parallel when paired with a suitable power supply and power filtering capacitors. Heat dissipation on the negative feedback shunts and power delivery are primary concerns.