Subwoofers driving me nuts

Placement + Bass traps. I recommend the Soffit Traps from GIK acoustics.

Try adjusting the phase relationship of the subwoofer to the mains.  Could be sub is substantially out of phase and cancelling at various frequencies in the low octaves.  While there is no exact solution either physically or in time (phase) in a room where there are thousands of reflections you may be able to minimize the cancellation and create a better overall balance.  To "see" what is going on I recommend using a spectrum analyzer, preferably one with constant percentage bandwidth filters, 1/3 octave minimum or 1/24 octave is better.  Very narrow band usually labeled FFT spectrum is also useful but can get confusing.  There are a number of free programs out there that work great for analyzing room acoustics.  Below is a link to some of the best I have found.  I think you will enjoy being able to see a little bit of what's going on and correlate that with your subjective impression.  Once you get a picture of what the room and speakers are doing you can try some of the many corrective solutions that are available.  Let us know how it goes.    
https://listoffreeware.com/free-audio-spectrum-analyzer-software-windows/
 

I can't hear the issue, that's the problem, right?  You have any room treatment, at all?  If you could get your position closer to a back wall, then treat the wall behind you. Normally  further from the speakers, the deeper the bass, to a point. Speakers 4-8 foot from (their) rear wall, 2-4 ft. from sides. Your position 8-12" from mains and 4 feet from your BACK, not the middle of the room.
French doors, sound like a bad idea, until they are opened on a back wall, behind the listening position. 
The BASS, escapes, not reflects. The wall behind the speakers is very, important, every wall has an issue, the more you can tame (or get rid of), the more you can hear.  If you can't kill let the sound escape, you have to use it, "reflex bass".
The only thing left at that point is measure and EQ, HUGE bumps, and that will take care of some of the NULLS. Then EQ up to where you need. In the setting position. If your trying to get bass on a dance floor, that's different too, four TOP corners, pointing down on the floor. or NO BASS.

If you can;  mains at 40-60hz and above, subs go from there down. THEN you'll probably start to hear more than one note, and bass.
Most people have their subs set WAY to high, Sub is actually what you CAN'T hear only feel. SUB.. You got to much of a GOOD thing.. I'm at 50 hz and down on two subs, and 35-40 hz and down on the others (2-4). ALL are cut at 20 and below. (glass is expensive).

Regards

Since the swarm-is-the-rage, I'm wondering how this config would sound: A 15-18 sub (single) in the middle, two 12s (left and right), and two 8-10s (further left and right). Another reason to play the lottery!

In my 28 year listening experience, I've never used more than one sub. I'm currently using a 20 year-old B&W ASW650 (lit says 12 inch but more 10-11 inch) in my smaller room (12 X 13). Listened to Joni Mitchell's intro to "Cotton Avenue" recently and Jaco's bass was "all there" which is good enough for me. 

Good luck with you project!