In the first scenario, each channel's low frequency data (under 60Hz from mains and under 80Hz from center) is added to the LFE track, but the LFE track itself is not subject to any crossovers and so remains unaltered. Likewise, no data is directed to channels other than the sub when a sub is present, so there is no overlapping bass (also called "double bass" sometimes).

In the second scenario, an external crossover could be used along with large mains and no sub to reproduce the behavior produced by small mains, but it would provide no benefit while also adding quite a bit of compexity - a separate component, a couple steps of manipulation applied needlessly to the LFE track, and some extra bouncing around for data crossed over from the other channels (center, surrounds). I wouldn't recommend it.