Rick, most(all?)HT receivers don't have a device to cut off the pre-amplifier voltage from the internal amplifier output channels when the pre-outs are used. This can be tested on any particular unit by inserting an RCA plug(doesn't have to be connected to anything)into the pre-out and observing whether the regular sound from the receiver to that speaker is cut off.

An example of where a cut off does occur is using the headphone outputs of most HT receivers. Since these units(contrary to some belief)don't use a "cheap" separate headphone amp, but instead the regular amplifier greatly attenuated for headphone use, the output would no longer be suitable for driving speakers. Therefore, inserting the headphone plug moves a mechanical switch which cuts off the pre-amp voltage from the regular output channels, which merely continue at "idle".

Phil, more than connecting a separate amplifier to one section of the speaker is needed for true bi-amping and to have any significant benefits. An external crossover(usually electronic)has to be used ahead of both amplifiers so that each receives only the desired limited range of frequencies to amplify. The internal crossover in the speaker has to be removed, or at least bypassed.

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Enjoy the music, not the equipment.