Firstly, does this receiver have a 5 channel stereo mode? It sounds like the auto function is switching to standard 2 chan. stereo, but manually your switching to the 5 chan. mode.
Secondly,I'm not sure I quite understand your dilemma.For your original settings,( Large mains, small- all the rest ) your receiver is working as it should for stereo listening.Only sub 80Hz bass from the center and surrounds plus any LFE info will be routed to the subwoofer.
Now, if you want your sub working full time then, as Bill stated above, set all your speakers as small.
As far as controlling the Xover point of the sub preout, you can't. The Xover is controlled by the receiver, set at 80Hz. If you want more control over the Xover point you could put another sub inline with your mains with the receiver set to large mains. Then the Xover adjustments on the sub itself will be controlling the Xover frequency.
How will this affect the LFE? The LFE info as well as the sub 80 Hz info of the other 5 channels will be routed to the "large" mains with the inline sub.
