I can offer a couple ideas.

1) ICBM. It will give you very flexible control of the crossover point, and it will keep the wiring fairly simple. In that scenario, you'd be talking about stereo interconnects from preamp to ICBM followed by stereo interconnects from ICBM to amp and a single interconnect from ICBM to sub. This is probably the "best" approach.

2) In lieu of the ICBM, you might look at something like Paradigm's X-30 control unit. I don't know what an X-30 goes for, but it could act just like the ICBM above.

3) Stereo interconnects from preamp to sub, and then from the sub to the amp. This was your initial idea, and is the best approach to take if you don't have an ICBM. The sub will be responsible for crossover work, so you'll need to get to the plate amp to make adjustments. You'll also be looking at some long interconnects, as you've noted.

4) To safe some interconnect mess, you could split the signals from the preamp and send the signal to both the sub and the amp. There is a chance for some signal loss at the splitter (how much is pretty debatable), and you will be delivering a full-range signal to your mains -- no crossover will be in the signal path to strip away the low frequency that the sub will be responsible for. The sub's crossover will only affect the sub in this scenario. This may be the cheapest and simplest, but not necessarily the best.

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