Thank you for the response, but I believe you may not have a full picture of how the amp and the subwoofer crossover work...
My amplifier has but one pair of speaker terminals. Even if it had "A" and "B" terminals, the both of them are driven by the same output transistors. Therefore, if you connect an 8-ohm load on the "A" terminals and another 8-ohm load on the "B" terminals, the load presented to the output transistors will be 4 ohms. Only by selecting one or the other of the speaker outputs, but not both, does the load equal the impedance of the speakers attached.
Secondly, the Definitive Technology sub does not have a "pass through" for the main speakers. Instead, it has a passive, adjustable network designed for 8-ohm speakers that can set both the main speaker high-pass frequency / level and the subwoofer low-pass frequency / level. This additional (and inexpensive) set of crossover components would interact in unpredictable ways with the speaker's own internal crossover providing audible differences. This is not only true theoretically, but I can hear the difference with my own ears.
The only idea that I've come up with is to build an "impedance adapter box." By inserting resistors in the hot wires of the integrated amplifier's speaker outputs, I could create line-level variable sources to drive the subwoofer's right and left "preamp inputs." The resistors would preserve separation between the right and left channels, and the high resistance would interfere neither with the main loudspeakers' crossover components nor with the amplifier's output section. In fact, Paul Klipsch, himself designed such a network to provide a common "center channel" driver for a center power amplifier.
Thank you again for your response, and if you have additional ideas, I welcome your feedback.
Cordially - Boomzilla