One iconic black box has probably more than anything else come to define the sound of rock - the Marshall amplifier. It has been, quite literally, behind some of the greatest names in modern music. With contributions from rock legends like Pete Townshend, Lemmy and Slash, plus an interview with the 'Father of Loud' Jim Marshall, this documentary cruises down the rock ages with all the dials set to 'eleven'. (bbc.co.uk)