Subwoofer matching

@kevemaher , Excellent analysis. It is very hard to impress on people just how wild the frequency response of their systems can get. I have seen worse than +- 10 dB. Measurement systems are not all that expensive and they can lead to substantial improvement. 
I beg to differ only on a few issues. From a measurement and functional standpoint subs are better off in corners and against walls. They are up to 6 dB more efficient in these locations and there is less room interaction. I think the problem for you is that your room dimensions are not dissimilar enough. You picked up on the wall effect and moved your listening position accordingly. I would measure at a few locations just behind the spot you are in now say at one foot intervals and chose the location that requires the least amount of boost from Daro. Not only does boost waste amplifier power but it increases distortion also, another reason to keep subs in corners. As long as you can correct the delays you are in business.  
As you note, you can not properly integrate a subwoofer and take advantage of the salient beneficial effects on the main speakers without a proper two way crossover. 
Look at the frequency response of the individual main channels and how they differ across the audio spectrum. It is not necessary to have both channels perfectly flat but it is very beneficial for imaging if the channels are identical or as close to identical as you can get. My own goal is always to be within 1 dB of each other from 100 Hz to 10 kHz. In order to do this you have to be able to measure frequency response and be able to EQ the channels individually. This can be done digitally without added distortion. The effect on imaging is easily audible. This also allows you to adjust the overall frequency response to get the tonal balance you like.
Proper room design and acoustic management are very important in spite of having full frequency room control. It saves power and lowers distortion. Unfortunately, most of us do not get the opportunity to build a room specifically for audio purposes and I am not a big fan of "bass traps." The best way to minimize that rollercoaster effect in the bass is intelligent room design, an unfortunate truth. If you can keep the swings to less than +- 5 dB you are doing a great job. 

mijostyn

... From a measurement and functional standpoint subs are better off in corners and against walls. They are up to 6 dB more efficient in these locations and there is less room interaction.

@mijostyn I have to believe that some of the LF problems that plague your system can be traced back to some your fundamental misunderstandings, of which this is just the most recent example. Subs placed in corners have more room interaction, not less. That’s how they produce higher gain from the corners, which certainly yields more bass, but not necessarily better bass. You seem to be a member of the "more bass is always better club," which explains your flapping woofers and need to use a rumble filter.

A thoughtful and nuanced explainer on subs and the corners:

https://www.axiomaudio.com/blog/subwoofer-corner-placement/

Room treatments are not for everyone. They are usually not very pleasing to the eye. But they do work. I have corner traps. I tried the "clap" test with and without them. The echo disappears using the corner trap. It is truly amazing. You cannot remove room echo (modes) without some form of room treatment. Curtains or wall hangings (canvas art?) will help suppress the mid and high frequency modes. But to stop the lower frequency room  modes, the resonant frequencies must be absorbed at one wall, preventing back reflections.from happening.
DSP will not fix this problem. since they do not stop room modes. Only physical objects placed in the room at appropriate locations will suppress room modes.