Jason, your understanding may not be entirely correct. When a coaxial cable is used from the sub output on the receiver, the receiver can handle both the high frequency rolloff of the sub and also the low frequency rolloff of the speakers quite accurately. Typically the sub is rolled off at 24dB/octave above the selected frequency(e.g. 80Hz)and the speakers are rolled off at 12dB/octave below that same frequency(with an accuracy of about 1-2Hz). When a high level connection is made with speaker wire the receiver does no bass management. The internal crossover on the sub only rolls off the sub, it doesn't roll off the speakers(even if the speakers are connected through the sub) as the receiver bass management does. Also, the markings on the sub's internal crossover control are typically far less accurate than the setting on the receiver. Some subs do have a separate fixed filter around 100Hz on their speaker out connections, but otherwise the speakers would run full-range and not be relieved of some of the low bass burden.
In short, for more accurate bass management which will affect both the speakers and sub use the line level coaxial cable connection and set the speakers small.
