Easily the most indepth look at how subtractive synthesis started and how it works. From Reddit!
http://alijamieson.co.uk/2015/04/forgive-me-lord-for-i-have-synth-a-guide-to-subtractive-synthesis/
http://alijamieson.co.uk/2015/04/forgive-me-lord-for-i-have-synth-a-guide-to-subtractive-synthesis/