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

For starters, I would only use the 2 Rythmik subs to begin with. I think once you have these dialed in, you can add the REL sub to see what affect it has, and start tweaking from there.

If you download Enrico’s dialing-in document, you’ll find you need to set the Phase correctly, and Volume match your subs to your mains (pink noise and SPL meter required). Without doing these, you’re wasting your time with guessing. If you do this, you might find it easier to fix that early dip (use the extended bass filter to see how you can bump it up). Don’t adjust both subs the same all the time, experiment with one sub only, then set it back and try the other sub.

You should also load a graph of your main speakers only. I think you’ll find the same results as the graph you posted for anything starting over 150-200Hz (I’m not sure what the cut-off is on your subs). Basically, the subs aren’t going to fix this.

This is without any DSP, just location and adjustments on Subwoofer - https://imgur.com/a/G9kHdTt

I had to back off the low end frequency (20Hz - 30Hz) because my wife was complaining it was making the house shake too much.

If it were me going through the time and expense of trying to setup multiple subs I would likely use a minidsp device to do it properly. 

For everyone saying 

“to do this right you need X” 

He has a mic and software with subs that have a low pass control. That’s all you need. Nothing else fancy especially a dsp with converts analog to digital to analog again. 
How much did you spend on your DAC and you’re trusting the DAC inside the DSP to reinterpret it all?  
All those extra things are for people that want the easy fix without actually gaining knowledge on how your system works together.

Again doesn’t need anything new (I understand people love to throw money at a problem but it could be fixed with what he has before the shipment arrives); just spend a little time with the low pass filter while looking at you readings from the mic.

Minus 5 db is not a null. It's a small dip. A null is an infinite dip. Your problems seems to be more in the high bass/low mid. If you can trust your measurements.....

Thanks to all for your useful posts. I appreciate them.

@golfnutz Good point — it seems like you’re suggesting start with one — using Enrico — then go to two, then three. I have the tools needed to do what you suggest. Comparing to the naked speakers is a good idea. This is what I’ve done in the past. (I have been doing REW sweeps since July of last year, including with moving the sub. At one point, I had the sub 4 feet off the ground to check for vertical effects.) I like your SPL graph. As for shaking the house, isn’t that our duty as manly-men-with-subs? ;-)

@mijostyn  I do really want to do this right. Because I just bought all my tube gear and powered subs, I will probably wait before buying a digital preamp with digital bass management capability and then selling off my active subs for passive subs. I see that if you’re right about this, I’m going about it the wrong way. But for right now, I’m looking for solutions to what I currently have. Down the road, perhaps I’ll change everything out for something you’re describing.

@mapman Mini DSP would be a decent experiment. I just have to absorb the idea of adding it to the signal path. It’s a question of which is worse — adding another thing or some bad curve sections.

@powemi I have a DAC I really like (MHDT Orchid) with a good tube and really nice upgrades by Grannyring. I really incline, as you say, toward trying to learn my room and my system. I want things to fit together organically, and I honestly enjoy moving a panel or twisting a knob and noting the change. I get in a headzone of iterative trials — I can go for hours, and all the while, I’m learning how physical space, quantification, and acoustic experience align. I would worry that a DSP would shortcut me out of this process. Still, I’m losing hair and if I pull too much more out…well…

@tvad Thanks tvad. I don’t know why Duke would want to advise me when I’m not buying anything. He’s got to be busy enough.