Nice set-up!
I bi-amped my 685s with my 1909 and it seemed to be better (although I don't have banana plugs, and by the time I'd re-configured it all, it's highly probable I imagined it!). In any case, for the cost of a few metres cable, it's a cheap thing to try.
I found a bigger gain (in the pure direct stereo mode) by upgrading my amp's power cord to a Clearer Audio Copperline. Also (if you haven't done so already) try your amp with all of the audessey 'enhancing' setting turned off. They definately made my system sound muddled and caused lots of distortion on the rears.
Finally, perhaps consider a rug in that room - you seem to have a lot of hard surfaces!