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

@hilde45 You don't need an EQ for your +5db hump (and that is actually a work around not the actual way to deal with it)

The hump occurs because the frequencies are being summed from your subs and your speakers (which will be different for each sub/speaker/room combo)

Your Low Pass for the sub needs to be lower as I am assuming that you are not going to rewire your crossovers in the speakers. If you have it at 80 currently (as another person suggested) I would say lower it to 55 and remeasure to see what happens. If it turns into a slight valley turn it up to 60. 

@hleeid I'm convinced I'll do a DBA, I just can't do it yet. I'm in a temporary room and will probably be able to move to another but not for 5 years. This makes a DBA impossible. I'm doing my best to optimize with 3 subs. If that is still 1 card short of a full deck (i.e., crazy) then I'll realize that and in 5 years will be able to correct the issue by selling the subs used and going for an array. Given the wide range of controls on these Rythmiks, I'm hoping I can do pretty well until I get an array.

@jmalen123 and @powemi2 Thanks for the suggestion process for the low pass.
@mapman, thanks for the Ohm stuff. I'll take a look.

FWIW, here's what I was able to accomplish after 6 hours yesterday:

5db null at 33 hz
6db peak at 77 hz
5db null at 155hz
5db null at 174hz
9db null at 208hz
10db peak at running from 379-594 hz

I put the graph on my system page: https://systems.audiogon.com/systems/9064/edit

Mediabridge Ultra Series RCA Y-Adapter (12 Inches) - 1-Male to 2-Female for Digital Audio or Subwoofer - (Part# CYA-1M2F-P) https://www.amazon.com/dp/B005DL2OVE/ref=cm_sw_r_cp_api_glt_fabc_ECCNFD1MD18K30G5AJ96

hilde45, if you really want to do this right you buy a digital preamp with digital bass management capability like the DEQX Premate or Anthem STR. You can set your crossover point and slopes independently then set delays so the subs match your main speakers in time and phase. Now you need only passive subs which opens up a whole range of DIY capability. You can actually build a better sub than a commercial unit because problems like shipping weight are no longer an issue. There are great sub drivers available. Otherwise you just bypass all the cheap front end garbage in the commercial subs and use just the amplifier section. But, outboard amps are much better. If electronics are so susceptible to vibration is putting the amp in a vibration machine a good idea? 

For this project, I can't think of a better resource than Duke LeJeune. 
http://www.audiokinesis.com/contact.html