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.
In the past year, or so, I've read about slimline "on/next to wall" powered subs -w- onboard EQ that might be an easier solution.

A quick search turned up models by REL and KEF, but the name of the well reviewed one (which initially got my attention) escapes me.

Anyway, here is a link to what one looks like.


oh hi DeKay. interesting, thank you! 
I was thinking this would be a process of measurement and refinement, and having a phase dial or separate amplifier might help me in the long run.

Yes, bad example (just wanted you to to see the size).

Here’s a self powered one by Gallo with phase and crossover controls.


Thought I read about REL’s, but didn’t see any when I went to their website.

Just something to look into.

Here’s a piece I wrote on how many ways there are to getting more bass.


And my thoughts on S v. P?  The only correct way to do this is in parallel.  Here's another thing I wrote which explains why:

Hey Erik, great reads. parallel is very doable, my amplifier has two speaker terminals.

As for improving in room bass response, I can work on that. I currently have a healthy amount of absorption. As mentioned, my projector screen is a giant baffle stuffed with rock wool, sitting behind/between the speakers. I back stuffed my ceiling coves with rock wool as well. those are steel lath/structo lite coves covered in lime plaster. I would like to think that I am taking some reflections with these huge curves.

might try more plants, they make for great diffusion, but this really isn’t the sunniest unit and gardening is a whole other forum. 


I’ve considered adding perforated helmholtz panel behind the black fabric. And now that you mentioned attenuating the higher frequencies to leave the bass behind, that could be my next move.

what about more (or less) toe-in? 
Hey Erik,

im still working this room, as well as the Zu Druid gap height. Their response becomes much more linear when they are near-sealed, a minimal gap height. I was going through different sweeps in REW with my UMik and well, it’s a mess. This is as linear as I could get with today’s tweaks.


This one here where I was almost sealed. As you can see, I’m also listening at pretty quiet levels, which will always adversely affect my linearity. (Neighbors, plus my own hyperacusis). With the low gap height, the bass below 100hz is almost gone, a configuration that may favor augmentation.

Going on your advice, with attenuation of frequencies above LF, I have a considerable midbass hump ~250hz-1000hz. I found these helmholtz panels by Decustik, and this one attenuates the most within that band. Having that my my giant 45” x 82” floating rock wool stuffed projector screen will hopefully tame some of that. Although if I’m correct, taming higher than 500hz with treatment behind the drivers is a bit too high to be omnidirectional, correct?