Tom-
The good news is you have HALF the crossover functions you need inside the XTI. The bad news is you still need the other half.
Buy a crossover, send the low output (set around 100Hz to start) to the XTI (ignoring it's internal processing) and the high output(s) to the Eons. A decent basic analog crossover (dbx) is $150 or so. You can also purchase a "loudspeaker DSP" like dbx DriveRack (PA, 260, 280, etc), BBE, Peavey, Ashly, Xilica, BSS, Klark Teknik, EV/Dynacord and several others. All are vastly more complicated & more expensive (but more effective) than simple analog crossovers.
If you re-purpose the Eons, you can get another XTI and use the internal processing in conjunction with your existing XTI to form a crossover, and the new XTI can power your new top boxes.
It's possible that simply getting a proper crossover in your rig (with some help, perhaps, to guide you in setup) will fix much of what you have problems with right now. How satisfied you remain is harder to predict. Loudspeakers have improved significantly in the last 10 years.
Tim "Gear Acquisition Syndrome Counselor" Mc
ps. We have a couple of general guides to gear that are often repeated. "The wrong piece of gear at the right price, is still the wrong piece of gear." It works nicely with "Buy once, cry once."
pss. This is your lucky day. What you need is in the limo.. er Marketplace.
http://srforums.prosoundweb.com/index.php/t/48767/143/ This is EXACTLY the configuration for your setup. Stereo in, summed LF output for your mono sub, the HF outputs to drive your Eons left & right.
psssssssssss. That crossover has been sold, but you can find them at any dbx dealer (try local!) or new & used on FleaBay.