Note: This article has been updated with lastest available information on January 13th, 2022 at 08:32 pm
The “Master” knob controls the general dynamic output of the current amp you are modeling – it’s the over all output setting for the amp and effects you have chosen through the app (the digital and downloadable part of the tones you are using).
The “Output” knob controls the general volume and output of the Spark 40 amp itself (the actual 2x20watt power amp section that creates the sound coming out of the speakers).
So, Master=digital modeling output, and Output=Spark 40 amp power output.
(Answer provided by Chris V. on the Spark Amp Lovers Facebook group)