Subwoofers is 5 is the Magic number?

First I have a terribly small listening room 15 x 17 x 8 foot tall feet with a large 7 foot by 7 foot opening on one side and large glass windows on the other side.

I really dont have any room treatments other than my Australian Sheppard that tends not to move when Im in critical listening mode. I have been adding subwoofers to cancle or excite room nodes and act as room treatment via cancellation or addition.So far its been a mixed bag from Epic failure, to Damm that sounds good but not real, to Hey that sounds about right!

For me I am now settled in with 5 subwoofer's. I have read for years that room acoustics is at least 50 percent of what you listening to and now I understand all the old post Ive read. Oddly enough the 6th made Subwoofer made it sound worse no matter what I tried. Possibly  a 30HZ hard cutoff would work not sure. The 5th Sub is in the most ridiculous position ever at the top of a 80 inch tall Armoire aimed at the ceiling. lol. Im running 4 of the subs in stereo front and rear all the same subs at the same volume and crossover (with volume pots for left and right) via a stereo AMP designed for the subs with one lifted 28 inches off the floor in the classic "Getty distributed bass array configuration" and the 5th Subwoofer in mono summed left and right with a dedicated amp. This is sounding best so far. I have a mini dsp that Im about to add to the mix. My Pass Aleph P pre amp allows me run the main speakers via XLR (not high passed / full range) and the Sub amps of the RCA's concurrently.

Side note I just bought some Pangea cables from Audio Advisor their SE edition for both the XLR and RCA's. Props to George Cardas Very nice cables for a reasonable price. I also am from AZ so I love the finest Copper in the world from AZ they use as marketing propaganda ... haha. They are made Build quality is excellent and sound very good for the price.

Five is magic for me too. Mine weren't near the placement problem yours were but then my room is more like 17x24x9, and its not just the room its the listening position as well. Everything in my room is spooky good dialed in for the sweet spot. 

Had one Talon Roc sub for years. Replaced that with four 10" subs and 2 Dayton sub amps. Was going to sell the Roc but just for kicks put it back in for a test and its not going anywhere! 

If you experiment I think you will find stereo/mono makes no difference. If you find any stereo bass let us know, nobody has, I sure haven't. That's not to say the bass doesn't sound and localize as if it was stereo. It certainly does! In spades! But every bit as good mono as stereo. Tried it both ways. 

I would never subject my precious stereo signal to DSP or any EQ. But if you can EQ one or more subs separately without polluting your stereo signal then with 5 subs and enough time and energy you should be able to get near ruler flat response down to as low as you want to go.

DBA Rules!
Stereo for my setup is a HUGE difference... I think because Kinergetics are crossed over so high at 100 HZ. Ive had a lot of Subs and never went over 60 HZ usual around 35 to 50 HZ. So I think these are weird ones. I tried to cut them low like I would with any other subs but the Compusound AMPs that were designed in the 90's for them work the best. It makes me cringe with the overlap cause I know Im Getting solid down to about 56 HZ with my Dunlavys. But it just sounds right. I have heard great things about Talons. I always wanted a pair of the Talon Ravens but they seem to sell as fast as they are posted. 5 is the number so far!
Well I have a pair of Talon Khorus I could sell you. The Roc sub is an isobaric design. Which back when it was bought seemed like the answer to "fast" bass. Now its clear fast bass is not due to a woofer being fast but to having a lot of woofers. I would bet removing one of the woofers from inside the Roc and putting it in another cabinet would be better.

Yeah your crossover is high enough there probably is stereo in there. Obviously, since you're hearing it! Lol! 

IME if you cross over subs at 60hz or higher, running them in stereo will benefit the sound stage assuming that the subs are place in a complimentary arrangement with the main speakers.  Very Low frequencies are typically summed into mono during the final; mixing process.
Our ancient Labrador with a hint of long hair flat coat retriever is an excellent bass trap, highly immobile until she needs out.

i wouldn’t throw out the Dunlavey just yet ...

are you a wanderer or tuning for a specific sweet spot ?

download a free copy of Vandertones and get the recommended SPL meter, it’s not a third octave game when residual rooms are concerned. Your opening is actually helpful, the low 8’ ceiling not so much - but you knew this.
the window is not mucking up the bass

have fun !!!!! Good dog !