The Marco-boy main system, as of August 2020

[Light Dependant Resistor]

So how would I adjust the input sensitivity,Marco.

Ideally all equipment we use, should match the level of the source media we purchase or use.

If used with a passive, then there is no need to change the sensitivity with a 306, as it then is perfectly suited to line level output from almost every known source component. This is because the RMS level in media we use rarely exceeds 350mv , The source component equipment can have, and usually does have much higher specification, but the media itself ie CD's Flac files etc in my experience never exceeds the 350mv mark, see attached which is a flac file being read by Audacity, the RMS level is in light blue

Level changes away from the source level, appeasing other equipment's lack of sensitivity,( IMO then on entirely the wrong path ) is then neatly avoided.