I was flipping through manuals etc.. for various EP600 vintages and AFAICS all of them have an RCA input as well as the XLR input. Maybe you have one of the very early ones ?
That said, one thing I missed was that all of the EP600 versions seem to only have a single mono line-level input (ie not left and right) while your pre-amp has separate left and right outputs.
In principle all you need is a dual-RCA to single-RCA stereo-to-mono summing cable with mixing resistors between the pre-amp outputs and the subwoofer input, but AFAICS nobody seems to actually make such a thing. Closest I could find was this
.Using a splitter cable to merge left & right is considered a Bad Thing here, because it connects the pre-amp outs from left and right together without mixing resistors. Found multiple reports of people killing their electronics that way.
EDIT - Awesome, thanks for the tip about RCA to banana. I put that string into Google search and came up with a bunch of hits from Amazon and others.
MORE EDIT - found a good page talking about the details of what you need, including a note about the impact of the summing resistors on channel separation IF the two sets of pre-amp outputs are just wired together internally rather than being driven separately. http://www.rane.com/note109.html
LAST EDIT - there is still an easy solution, which is to run speaker wire from power amp outputs to subwoofer high level inputs... as if the subwoofer was a pair of speakers that you were hooking up in parallel with your "real" speakers.
You could probably have banana jacks on both ends
But that would take all the fun out of it.