Dan, you're correct that one possible benefit of using a sub is to relieve the mains from trying to reproduce low frequencies which they can't do cleanly. However, this can't be accomplished in the way discussed here. A sub doesn't have a "high pass filter"; the control on the back(sometimes incorrectly referred to as a "crossover"), is only a low pass filter which rolls off the sub response above the selected frequency, but which can have no effect on the response of the mains. This is the case regardless of whether the mains are connected directly to the amplifier terminals or are connected to the speaker level outputs on the sub(which are simply straight-through connections to the amplifier).
Relieving the mains of the bass burden requires a multi-channel HT receiver or separate processor having bass management circuitry, with the mains set "small" at an appropriate frequency, typically 80Hz. The sub would then be connected to the sub out with coaxial cable.
