How do you use a sub with active speakers

Why do I not want to sum the left and right? Isn't that effectively what the REL does when connected through its high-level input wired to the left and right speaker terminals?

And thanks for your help, guys.

Let me answer some of the questions raised:

The pre is a GamuT D3

The sub is a REL R-205, which has only one low-level input

Samujohn: I'm not following the relationship between the balanced speaker connection (which fixes a hum problem I had running unbalanced) and the sub puzzler. Will the sub problem be resolved somehow if I connect the Quads unbalanced?

What I am doing currently is running the sub off one channel only. That's obviously not a long-term solution, but I'm amazed at how well it works, and the REL and Quads go together very well. I'm reluctant to throw more money at this with something like the Velodyne box unless I decide to stick with the Quads. I was really just hoping to find a workable solution to get up and running. If I were to spend some more, another approach I would consider is getting a second R-205 to run from the other channel. Stereo subs are supposed to be great anyway.

I appreciate everyone's comments, thanks.

Interesting idea, Al. It might work.

On second thought: while the LFE has its own volume adjustment, it does not have a crossover adjustment and doesn't utilize the crossover setting for the hi/low level input. Because it is intended to take the purpose-made LFE or .1 signal from HT, it runs full range. That wouldn't be good.