I don't seem to have the z-offset option since the first switch on. Also I see that in the build manual it references the forums when mentioning the Z axis numbers going all over the place when driving the axis via the knob. Is it a case of flashing some new firmware? For ref: BigBox Dual.
Dual has not the z offset. You gotta go to the dual section in the build manual... http://wiki.e3d-online.com/wiki/Printer_Commissioning#Dual I do not have an idea which version is flashed on your RUMBA but a update at this age does not hurt since only the configuration got refined...