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

@mapman -- I'm not against it, absolutely, but want to see if I can make a casserole with what I have and with the skills I have. I've spent a fair amount to have more than decent cables and DAC, so to add something into that mix is not necessarily about "polluting" the signal so much as adding an element that will change the character in unintended ways, even as it may be correcting other outcomes. But I hear you. Not ruling it out.

@tvad  That's a good object lesson. And I'm not aiming for Howard Hughes levels of cleanliness in line response. My issue is that I can hear the bump that I see measured. I tested that out last night -- I found a track of acoustic bass on a Chesky recording that goes up and down the scale, knew instantaneously where the muddy/tubby section was, measured the frequency of those notes on an analyzer and then compared to my REW graph. Exact match. If I can at least manage a couple unwieldy peaks, I will not have lifeless music, but I will eliminate flaws which my ears can hear. That's the goal.

That said, we all remember Hawthorne's short story, "The Birthmark," where his otherwise beautiful and kind wife has a birthmark which the protagonist insists on removing? Spoiler alert: the process kills her.

@tvad 
I believe that was very good advice from the manufacturer. No dsp software is perfect but you won’t succeed without. You may think so though...

So in my case, FWIW, I’ve been able to get the sound I want with a sub and good but not full range small monitors (KEF ls50 meta) simply by getting the mains setup optimally first, then using decibel app on iphone and streaming white noise to get the sub to visibly fill in the missing lowest octave. Only three controls to play with on one sub, a Klipsch sw308: level, low pass cutoff, and phase. I eyeballed it with the meter app, then sat down with some music to fine tune the sub. It’s perfect to my ears now! Took about 30 minutes to get it dialed in (sub controls are accessible from main listening position). Discovered I had not landed anywhere near the right level and settings for the sub by ear alone. Low pass cutoff way too high and level way too low.

It helped that that room is thin dense carpet pad over solid concrete foundation. No floor resonance issues there like I have upstairs to deal with. If you have those, tackle those directly with isolation pads or equivalent under speakers.

Of course all rooms and ears are different. Also experiments are how you learn. I learned never make these things any harder or more complex than necessary to deliver results that are technically good but more important pleasing to the listener.

NExt step would be DSP just to see what happens and if worth it, but I am at a good place now so not much incentive to dabble more than needed at present. Maybe someday when I retire.