Subwoofer matching

Chilli, Any subwoofer that can match the volume you like to listen at will be fine. Most of us rarely listen over 95 dB and any pair of 12" subwoofers will do that easily. They all have adjustable gain. You just adjust it to match the output of your main speakers. It will require more power to reach the same volume but all the decent subwoofers are equipped to handle that. The 4 10" drivers you get with the swarm system will also do just fine.
So, unless you listen at 115 dB any decent subwoofer can be made to match your system. You are right to start minimally with two. 

Kalali, the idea that a smaller subwoofer is faster is incorrect. Given appropriately designed motors a 15" sub is just as fast as a 10" sub. The 15" driver does not have to move as far to produce the same output so typically larger drivers have lower distortion not more. When a driver can not react to the signal in time what happens is the driver's response falls off or they start "breaking up."  Most large drivers can go way higher than they are required to do. My 12" drivers will make it up to 700 hz before breakup and I only run them up to 125 Hz. Now you can get away using smaller drivers by using them in multiples. This only makes sense when you have to get the speakers within a certain form factor. Can't put a 12" driver in a tower speaker 10" wide. Otherwise, when it comes to low bass large drivers rule. For most residential sized rooms say 16 X 25 four 12" drivers will do the job by which I mean 105 to 110 db comfortably. 

@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. 

@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.