I have split out the files into separate zip's, saves downloading one file just to get the Arduino code (for example). I haven't included gerber files, mostly because they can easily be generated, however would they be useful?

The electronics zip file now has the 3 circuits to be able to build the amplifier. My next thing is to write a BOM for easier ordering as the circuit diagrams don't show the additional parts needed (RCA connectors, rotary switch etc).