Zu Druid V passive subwoofer augmentation

Hi friends,

my current loudspeakers are Zu Druid V’s and I am looking to augment my bass output. I know that a 10” wideband can’t compete with 12 or 15” LF drivers. I Have 12” LF in my bedroom, and I used to have 15” drivers in my main system. But the Zus are here to stay.

I live on a top floor pre-war apartment, with a very good working relationship with our downstairs neighbor. The Zus are are sitting on a 7/16” rug pad underneath a wool rug already, muffling their downward port. I really don’t want to have a rap for loud music. I didn’t pull up my flooring and insulate below the subfloor when I renovated, but maybe I should have 😂.
To get a deeper frequency extension, I was just thinking of additional bass drivers. At my low SPLs, the loudness curves are going to leave me with less LF data.

On the wall behind the speakers I have a 91” black projector screen, built as a frame. (45”x 80, super easy project). It’s stuffed with rock wool, 4” thick, and spaced off the wall 3”.

 With my modest real estate, I could rebuild and reinforce my screen to accommodate these. https://www.klipsch.com/products/pro-1000sw

I was first considering getting some kind of HT pre/pro that would handle the XO and getting a discrete amplifier to drive them, likely a used Denon AVP A1HDCI processor. But I just purchased some Denon POA-S10 monoblocks and their matching PRA-S10, which is clearly a 2.0 setup, and I would prefer to use them if possible.

My thought was to run everything wide, with the subwoofers inverted polarity. From my listening position, the subwoofer drivers would only be 12” further away from me than the 10” wideband Zu drivers. It would situate the subwoofers about halfway between my ceiling and floor in terms of their height. 
The Zus have two terminals, Speakon and binding posts. They are both live. I’m currently running them off the Speakon connector with the optional 25Ω Resistor across the binding posts. My thought was to have the amplifier attached to the binding posts, remove the series resistor, then to have the Speakon output going to the subwoofers in series with polarity inverted. That would give me the combined impedance of the Zus, plus Klipsches 1/(1/16Ω+1/8Ω)=5.33Ω at the amplifiers. I’d also lower my Zu gap height to get a more linear roll off.
I believe this should be okay for POA S10s rated at 150W into 8Ω, when I’m using less than 1W right now, listening to Elysian Fields on my Druids as a type. 
My goal is not to get ruler flat, but just a deeper, albeit rolled off, augmentation of bass at low SPLs. I don’t want my Zu’s downward port freaking out my neighbor’s MaltiePoo.

what are your thoughts on series versus parallel? Inverted polarity for wideband subwoofers? Wideband versus the active solution? The Klipsch In-walls in my screen? Take in mind, building something into my screen is basically my only option, in terms of real estate.