there's no such thing as "line" or "mic" (switch) on the G3 sennheiser receivers, you can set the output level per 3dB. i don't see the point of +18dB boost on the receiver & then padding -18 dB on the mixing desk....
+18dB relative to what? dB is an expression of a ratio between 2 levels. If it says +18dBV it is relative to 1 volt, dBu is relative to .775 volts. Without a suffix it needs to have an explanation of what it is relative to in order to have any meaning.
I would be useful to know what is inside the G3 receiver. Is it an amplifier, or is it padding the signal to get it down to the lower level? In the world of digital signal transport where the mic pre is at the stage it may be less important to run higher levels between the receiver and the mic pre, but with long analog snakes between the receiver and mic pre you probably want to run as high a level as possible to avoid additional induced noise in the signal.
If running the receiver at +18whatever causes you to need to pad your console input 18dB then don't do that, but if running it at the lowest level means you need 35dB of gain to get good level, then you need to run a higher level.
Mac