I, also, would agree another BS Ultra would be the best way to go (and cost less as well).
However, have you spent any time listening to your system without the DefTech center.
It's free to try aside from the time to unplug your center, reconfigure your AVR, and run Audyssey.
IME, with good speakers like you have, it works very well; and may beat your current setup. Sure, get the Ultra BS or Center, eventually; however, while you are saving up, do a comparison of Phantom Ctr vs DefTech Center.