Luke has 8 sealed 21s and they still barely keep up with two othorns. Maybe if you put the sealed subs nearfield they will stand a chance. 0-30hz shouldn't be localizable at all. If they're 8-12 feet closer to you, that makes up a lot of lost spl to distance. Awesome stuff you have anyhow. Isn't there a processor available that can handle all of the work instead of those behringers?
I did expect the Othorns to easily overpower the Daytons, and I would love to be able to run eight sealed 21" subwoofers one day too. Even though they may not keep up, I do feel going further like with 21" will still help give me the boost in the infrasonic range I'd want. Since my room is above the garage, the impact will optimal with the suspended flooring.
I've considered nearfield as well, though unfortunately there's no way to accomplish this, I'm afraid. Unless I replace the bedframe and box spring with subwoofers for the mattress to be on top of. My bed is behind the sofa. I pretty well turned my room into my own little home theater space.
As for the Berhringer units, there are plenty of DSP options available out there, these just were the best bang for the buck when I got them. Right now they aren't even used to do anything for the LCR, just the LFE. Since I got the Marantz AV7703 I can take the DEQ2496 out of the equation since it's in bypass mode right now.
What I may do is take the LFE connection from the pre-pro and put it to the DEQ then digitally feed a DCX unit dedicated to the subwoofers. So that way I have all the control I would ever need there. Though I may not really need the DEQ still.