Like all ITT-KB radios, distortion is not normally a feature. Don't rule out checking the speaker for rubbing or loose diaphragm inner support.
The ITT Service information is also on the KB Museum website, it includes a printed circuit layout to help you locate the components.
http://www.kbmuseum.org.uk/itt_servi...omatic_ITT.pdf
Good luck, they sound good when working correctly.
Mike