It's best to experiment - as long as you keep the Output knob turned down so you don't hurt your hearing or anger anybody else nearby, play around with the knobs and listen to the effects they have and how they vary the sound.

The very small Music Volume knob beside the Headphone jack controls how loud the additional input through either direct cable into the Aux input jack on the back of the amp or through bluetooth, so that we can get the proper mix between whatever backing trax we want to use and the amp's guitar/bass output. The first control knob is important only if you're trying to use the amp without the app - you can select various default (built-in) amp models which the other knobs will then work on. Typically in an amp, the Gain boosts/cuts the signal that gets acted on by the rest of the amp and effects, and the Master controls the overall level of sound coming out of the amp, but the Spark provides this third level so that we can get an awesome tone through judicious use of the Gain and the Master controls but still have that awesome tone come out at a low volume so as not to disturb others in the building. The Output knob is the final volume adjustment which controls the level of the sound coming out of the amp. Each of those slots has its own level adjustment on the amp as well as in the app. The Bass/Mid/Treble knobs are for the EQ (such as it is) to adjust the tone, The Master is how loud/soft the signal is that is then sent to whatever effects (if any) are enabled in the Mod, Delay and Reverb slots in the effect/amp chain as established in the app. The Gain affects how much of the original guitar signal is fed into the effects/amp chain.

Each knob has a function that is clearly labeled. What's to know? You turn the knob, it affects the sound.

Positive grid spark amp