I don't see the crossover settings in the manual I downloaded, but that WOULD be a good way to set up the sound.

I've got to go read about that four channel mod... I THINK I see it on page 8A-19 of the service manual. I'd have to play around to see what pin is what, unless it's called out in the pin outs of the plugs in the back of the manual. Somewhere. In the +3000 pages.