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Actually, supposedly it is recommended that for the best flat response and mix of speakers and sub, you should choose a crossover at approx twice your spec'd low freq response of your speakers.

Yes. The rule of thumb I heard is the crossover should be at least twice the -3db point of the low frequency range. The -3db point may sometimes be more stringent than the low frequency range. Some speaker makes say the low frequency range is the -6db, -10db or even -15db point.