Subwoofer matching

@kalili, Speed is a silly term with speakers in general. You can talk about frequency response. A 12" driver going at 500 Hz is moving just as fast as as a 4" driver going at 500 Hz. Assuming both are doing this without distortion the only difference will be that the 12" driver will be more directional which may actually be a good thing. If a driver can not keep up it's high frequency response falls off. 
Your problem was a phase/time issue which is the most common reason people can not match up subwoofers. Here is a trick. Play a test tone that is right at the crossover point say 80 Hz. Sit at the listening position and have someone move the sub in increments until the tone is loudest. Try it also reversing phase on the sub. This is not a perfect method but it helps.
A sub can be in phase but out of time and the reverse. For a perfect match the subs have to be in time and phase with the main speakers at the crossover point. The best matching also requires a full two way crossover. This business of tucking a sub in under the main speakers without a high pass filter is just a cheap way of doing it created by the sub manufacturers to sell more subs. People do not like add on expenses.
If you can't get a full 2 way crossover you can always solder a capacitor in series with the input of the amp and roll the amp off at 6 dB/oct. Once you know the amps input impedance it is easy to figure out the size of capacitor needed. 

Here's some of my thoughts on rooms and subs.

Right now, I'm figuring out the best location for my system in my room. It's about time. My room is 14.5' x 19.5' x 8' high. I listen across the 14.5" dimension out of necessity. I am using a readily available Room Analyzer package. This hardware/software generates audio sweeps, records the sound pressure with a mic, and calculates freq response, reverberation time, room modes, phasing and delay times. I am using a DIY sub. It has a 10" Scanspeak woofer in an acoustic suspension design. The main speakers are B&W 805s on the stands provided by B&W.

With the listening position against the wall opposite the speakers and the sub to the left of the left speaker, I've found several frequencies (40, 62, 125, 175 Hz and others) where there is a peak or null with a long delay time, much longer than other frequencies. Hand calculation of the expected room modes fits this observation. At 40 Hz, the sound wavelength is about 28'. Half wave is 14', which is nearly exactly the length I'm listening across. So room modes are important for an average sized room. The 40Hz peak is +10dB (above mean). the others have similar decay times but are not as strong, although the dip at 63Hz is nearly -10dB.
I moved my listening position (sofa) out into the room away from the wall. I placed it 1/3 of the length of the room (recommended). New measurements are now quite different. All modes are suppressed to different degrees, but are still there. The software cannot find any modes, although I do have irregularities in freq response.
Listening position is a major factor in suppressing room modes. The explanation is that the new listening position is near a null in the 40Hz room mode. The conclusion is that room modes are important for bass. If you want good sounding bass from 30Hz upward, room modes must be considered.
Now, imagine I were to use the DARO system in my previous listening location. The DARO would call for 10dB suppression at 40Hz, but also for a approx +10dB increase in SPL at 63 Hz. This means that DARO would be calling for 100x more power from the sub amp (see https://geoffthegreygeek.com/amplifier-power/). This is very difficult to do for most amps. The DARO can reduce peaks, but cannot be used effectively to remove dips.

In order to effectively suppress modes, several things are needed prior to using DSP.
.First, get the sub away from the walls and corners. Get it (them) out into the room (unless you want your bass to overpower everything else).

Second, use the recommended listening position.
Third, after the first two are done and characterized, add room treatments that includes corner traps and bass traps for the low frequencies. These absorb the appropriate frequency bands so the standing waves are absorbed, not reflected. Additionally, the long reverberation will be dramatically reduced.
The first two modifications will get you a long way, but room treatment is absolutely needed to suppress the remaining modes, unless you are fortunate or the modes are sufficiently suppressed for your taste.

After the above three things are done, then the DSP (such as DARO) can be used as the peaks and nulls will be of lower amplitude and therefore the amp will not be stressed as severely.
In conclusion, room modes determine the lower frequency response of an audio system. The exact modes depend on the room size and geometry. Room modes cannot be corrected entirely with DSP software, especially nulls (dips).
The modes are there whether DSP is used or not. Optimal location of the sub(s) and the listening position is vitally important. To avoid excessive use of amplifier power, add room treatments prior to adding DSP.
Please note that there are other considerations if two or more subs are used as they can be placed such that certain room modes can be suppressed. But room modes are still present since these are characteristics of the room only . One person's solution may not work for someone else. Other modes may not be.suppressed. Room modes are always present in a room since these are characteristics of the room only . It doesn't matter what is in the room, the modes still exist. Any item added to the room can only affect the relative amplitudes of the room modes (this goes for treatments also). One person's solution may not work well for someone else.

Even without a room analyzer, one can improve the performance of an audio system. Searches will provide guidelines, such as placing the listening position about 1/3 (actually 0.38) of the distance between the walls behind the speakers and the listener..

Some boilerplate - I am not affiliated with any manufacturer of audio equipment, room treatments or DSP. I have no interest in selling you anything. I have an MS in Physics and over two decades designing and building optical systems (telescopes for example). Wave phenomena are very familiar to me.
Constructive criticism is always welcomed.

It is quite simple to "match" a sub to a very efficient speaker. If you can't turn the amp on the sub up enough, then I suggest you get a two way crossover. With this xover you will have independent control of the signals being fed to the main speakers and to the sub. You can obtain a calibrated SPL monitor. These are handheld and usually can be affixed to a tripod. You then can match the speakers with the sub. You can make fine tweaks afterward according to taste.

@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.