This video shows how to easily make a manual ABX switch box to test your friends. This video, Method 1, uses an inexpensive switcher that sells for about $20, "TENEALAY RC-32". It can test any line level products such as DACs, preamps, equalizers, or even just RCA wires as long as the two sources, A and B, are already synchronized and playing at the exact same volume level.
NOTE: The only reason you'd need a total of four Y-splitters instead of just 2 is because some super picky audiophiles will cry foul and insist that the wire terminated with the y-splitters, even though one branch of it leads to nowhere (electrically speaking) it will still supposedly "sound different" than the one leading to the switch box directly. They are wrong but it is not worth arguing with them so simply put y-splitters on the ends of BOTH 'A' and 'B' wires.