So there's no way to turn off the speaker's internal amp for the sub and use a jumper between the speaker's binding posts?
Then I suppose you could set the fronts to LFE+Main and set the surround rear channels on the receiver to bi-amp. Then the main speaker terminal could go to the regular speaker posts and the surround rear channels that are sending a bi-amped signal could go to the built-in sub's speaker level inputs. From your sig, it looks like you have a 5 channel system, so you should be able to configure it that way.
The only other way I can think of would be to divide the LFE RCA cable three ways.
I'm just throwing out suggestion here because it's pretty hard to find any info on these speakers to see exactly how they're set up. But I'm sure you already know that.