Setting mains to LARGE and sub to OFF will route bass from other small speakers to the mains.

This is a tough nut to crack. To run stereo subs in the way you describe (large mains and no sub in the 950, external crossover to steer audio between subs and mains), there's no easy way to get a signal for the transducer. If you turn the sub back on, though, the LFE track goes only to the transducer - which is hardly optimal. What kind of external crossover do you plan to use? Could you split the sub outputs and take the second left/right sub outputs to the transducer (summing them either before or at the transducer's amp)?