The Tone switch determines whether or not the tone control circuit is on or off. In the on position the Bass and Treble controls manage the tone. The off position makes the tone controls inoperable.
The Duplicate switch allows recording or copying on two connected tape decks simultaneously, i.e. duplicate a tape.
Download a service manual at the following link.
http://akdatabase.org/AKview/albums/userpics/10004/Pioneer%20SX750%20Service.pdf