What process did you use to integrate multiple subwoofers for 2 channel listening?

MiniDSP 2 x 4 HD, a calibrated mic ( Umik-1 or Dayton ), a db measurement tool of your choice ( decibel: db sound level app on iPhone is great ), REW app on your laptop, YouTube tutorial Episode 7 from "Hometheatergurus" channel ( along with the gain matching episode )  and you will get absolutely amazing results if your willing to go through the headaches of a learning curve. 
I have a Focal 1000F, Def Tech SC4000 and an old Def Tech SC-1
 ( how's that for a crazy ridiculous mix?!! ) and it literally sounds like I have a single subwoofer from the main listening position. The SC4000 is only about 5-6 feet behind the listening position so occasionally ( but rarely ) I have to adjust the volume but it has remote control and through my Harmony hub it's easy as pie to do.
  MiniDSP allows for 4 configurations ( again controlled through the Harmony hub ) that I have set from flat response to a couple of different mid bump options to play with depending on the movie or music track and my listening whim at the moment. 
  A long time issue I've dealt with from just one to two to three sub configuration is now a mute point. Although nothing is perfect, the technology available is a wonderful tool if your willing to give it a try. I guarantee it's better than any " by ear " method, I don't care how good your ear is or how long you crawl around your room. Your just finding bumps in frequency at different areas of your room and not flat response anyway . There are too many factors so let technology be your friend. You ( and your ears ) will be glad you did.

P.S. - room treatments would be a great addition to the equation. That will
         be my next step hopefully.

Update. After a lot of work yesterday, I’ve made good progress. I have no peaks over 4 db, and 4 nulls that are -5-6db and one big narrow one that is -8 db.

Scan results are on my system page:

https://systems.audiogon.com/systems/9064/edit

Basics of what I did:

• Started with a baseline scan. No subs.
• Assuming positions I had worked out before were adequate....
• Added 1 sub (REL). Adjusted in minute increments to (a) gain, (b) crossover, (c) phase -180 or 0.
• Added subs 2 and 3 (Rythmik). Adjusted in minute increments to (a) gain, (b) crossover, (c) phase, (d) various filter adjustments on sub
• Once those measurements were all optimized I adjusted:
• Front wall treatments, adding, subtracting, and moving.
• Side and other area treatments, same.
• Ceiling treatments -- doubled and adjusted position of existing treatments.

REW notes: Besides using REW to play sweeps and analyze the curves I tried something new (for me).

• I looked at the graph to identify the frequency where a peak or deep null was.
• Then, I’d play a constant tone and move a single treatment, facing the laptop screen display of the SPL meter, keeping an eye on the tone and an SPL graph next to it.
• When the the SPL graph showed a real decrease in a peak (or rise in a null), I’d leave the treatment there and then go back and do a sweep.
• Then, I’d repeat it and either shift treatment position or add more treatment there or nearby.

By doing this for the most problematic peaks and nulls, I was able to try new and more asymmetric combinations of treatment positions to really address the curve. In other words, I finally discarded my visual bias.

@hilde45
I'm curious, can you post what your settings are on the 2 Rythmik subs please, thanks.

@golfnutz Happily.

Working very well in my room:

1 REL Sub (further away)
Phase: 180
Crossover: 100
Volume: 7 p.m. (3 tics)

2 Rythmik F12SE Subs (both behind and to the outside of each main, but not exactly equally):

Both set at:

Phase: 180
Crossover: 40
Volume: 7 p.m. (3 tics)
LFE Pass filter: 50hz
Rumble filter: Off
Extension Filter Freq: 20
Extension Filter Damping: Mid

Thank You.