Mike, of course speakers roll off and don't suddenly stop, so whatever frequencies are still being output in the bass by the mains will be duplicated by the sub, not to a good effect in my view.
Your "Come to think of it..." is my view. If a sub is used, set the speakers small and set the crossover at an appropriate frequency.
No receiver setting could set the mains large but not force them and their amps to carry the low bass load because large means full-range by definition. The benefit of bass management in relieving the speakers and the receiver amps of part of the low bass burden requires the small setting.
Yes, if a regular stereo source with no .1 LFE channel is being played, the mains are set large and the sub is assigned LFE only, it should receive no output.
