http://libinst.com/Audio%20DiffMaker.htm
some of the tests the author suggests are suitable for the Diffmaker :
When you are interested in whether an audio signal is actually being changed by:
* Changing interconnect cables (compensation for cable capacitance may be required)
* Different types of basic components (resistors, capacitors, inductors)
* Special power cords
* Changing loudspeaker cables (cable inductance may need to be matched or compensated)
* Treatments to audio CDs (pens, demagnetizers, lathes, dampers, coatings...)
* Vibration control devices
* EMI control devices
* Paints and lacquers used on cables, etc.
* Premium audio connectors
* Devices said to modify electrons or their travel, such as certain treated "clocks"
* Different kinds of operational amplifiers, transistors, or vacuum tubes
* Different kinds of CD players
* Changing between power amplifiers
* General audio "tweaks" said to affect audio signals (rather than to affect the listener directly)
* Anything else where the ability to change an audio signal is questioned