yeah, that's not right.Using the G10 causes the tone to be a bit duller. When you switch back to a cable it's like you took a wet blanket off your sound.
if anything, the wireless should be brighter, because it eliminates the cable loading that darkens the sound of passive instruments. (enough that the better line 6 units have settings to reduce that brightness, that's what the "cabletone" options do.)
you might have a bad wireless there.