This is a common problem due to the way SMPS are built, they always have some leakage current from primary to secondary and when all equipment is unearthed you can feel it tickle you, it is usually not dangerous though (measure AC amps from ground to earth) it is quite annoying.
The answer is to earth at least one point of the common ground, but then earthing one piece of equipment only can run you into other problems when disconnecting cables between them: The typical example is having one piece of equipment un-earthed and one earthed, when connecting an RCA jack the center pin will touch before the shield (ground) sending the floating 100V or so you're measuring to the ground through an IC's input, that is sometimes enough to destroy the IC.