Seems that it is preferable to keep the tank horizontal if at all possible. If by dolly, you mean like an appliance dolly which would mean putting the tank up on its end, that would be my least preferential option. I agree with Jeff, don't really see any harm in a slow careful slide down the stairs, if you can handle the setting down / picking up at the top/bottom. Four people walking 700 lbs of glass down a narrow staircase sounds very difficult.