I'm sorry for my ignorance, but what do you mean variable vs. fixed? Is sub-out variable or fixed?
Thanks much for your response.
In a variable out, the volume control on the receiver, or pre amp, will also control the signal level of the out put. Thus the term "pre out" The sub out, I imagine, would be variable, as once you set the level of the sub, you'd want its volume to go up and down with the rest of the speakers.
A fixed line out is the full line signal and it's not affected by the pre amp, like the line out of your CD played.