Subwoofer matching

How many is much more important than which ones or how expensive. In other words, regardless of your budget its better to spend it on 4 than 1. Put another way you cannot find one sub that will be as good as 4 that cost 1/4 as much each. Search Swarm or distributed bass array.

System matching with 4 is a non-issue.

Recently I had great result by adding two 18 inch Scaena subwoofers to my Lansche 4.1 speaker.

You may want to contact Mark Seaton to get advice on subwoofer to add to your speaker.

https://seatonsound.net/product/f18-subwoofer/

https://www.whatsbestforum.com/threads/what-is-the-most-musical-fast-and-accurate-subwoofer-you-have-heard.21597/page-4

I'd suggest placing a phone call to REL and asking their opinion. REL customer service is very good.

Why not use the Avantgarde subwoofer?  It's specifically designed to work with their horn loudspeakers.

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. 

Aside from matching the listening volumes, in my experience the bigger issue is matching the ‘speed’ of the subwoofer driver(s) with your horn system. The suggestion of using multiple small driver subwoofers is much better than using one or two large driver units.

What deficiencies are you trying to address?You don't say whether the subs are meant to compliment the AG subs or replace them. If it is the latter, bear in mind this bit of info, as per Jim Smith years ago, that the AG Duo subs, (the sub 225 in my case), have "meaningful" output up to 500 hz. You could end up with a hollow midrange if your intent is to simply jettison the factory subs. I have no idea what "...negating a pair of subwoofers..." means.

I would do a little more investigating and proceed (or not) with caution.

I have 99db efficient horn speakers and use 2 RELs (bought used in mint shape for around 200 bucks each...a 10" and an 8") with 'em, and have never sensed a lack of "speed." They have input level controls along with phase and frequency adjustment...works swimmingly.

The quoted words of caution are from REL customer service.  My I am suspicious but they recommended two of the biggest most expensive subwoofers they sell.  The REL recommendation was seconded by a couple of friends.  I am also considering SVS subwoofer.  They did not seem to think that their subwoofer would have a problem keeping up with the AG Duo’s.

The reason I am not keen to buy AG subs is that I don’t want to have flexibility issues later if I migrate to a different main speakers.

Have you considered trying a pair of the HSU subs, they are reasonably priced and sound good with horn speakers.

I will look into HSU.

Acresverde, the goal is to complement the current speakers.  Not considering replacing the bass drivers that coming with the AG duo’s

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. 

My experience with matching subs to just about any speaker is to go with an array of multiple subs rather than just a pair, as others have correctly observed.  The idea is to have a large wave launch that can match the volume level of the much faster, smaller drivers without having to push them hard.  If you have you ever heard the Infinity IRSs they are a good case in point, as the ribbons in that array are extremely fast.

I have AG Trios and they came with a two pair of dual 10" inch powered subs.  That combination was just barely OK, and tended to fall apart at higher volumes especially with rock music.  I suspect that is why AG came up with the idea of horn subs, but those beasts are huge and expensive.  I solved the sub  problem by adding another pair of dual subs, now 12 total and it was trans-formative, not perfect but much improved.

You might want to consider just picking up two pair of used 225s.  They are relatively inexpensive on the used market.  Also, I agree with Mijostyn's comment regarding speed and subwoofer size.

REL customer service recently told me they won't repair either of my subs should they ever need it. Hmmm...also, "speed" is a silly term and I don't think it's applicable to subs or really anything else in audio. I think some bass overhang in reverberant environs might be what makes people think there's a velocity issue, and many think it comes from cone material...it doesn't.

I am in the same camp as those that say quantity of subs is more critical than quality.

I have four subs in my system made by three different manufacturers.

"...also, "speed" is a silly term and I don't think it's applicable to subs or really anything else in audio."

Perhaps in your opinion and in your experience, but not in mine. I have tried various different combinations of three powered subwoofers; 12" and 15" - one 12" I own in a different system and the other two borrowed from friends, with my Omega Super Alnico Monitors, and in all cases I felt the subwoofers could not "keep up" with the single drivers and there was an obvious tonal lag in the portion of the bass delivered by the subs. I and the man who designed/built the Omegas express this phenomenon as "speed", others might call it something else. His recommendation was to use subwoofers either with two 8" drivers in a sealed box or multiple 8" (or at most 10") driver sealed subwoofers.  Call it whatever you want.

got 80 cubic feet?

https://www.ebay.com/itm/6-Electrovoice-30W-sub-woofer-speakers-for-tube-amplifier-audio-western-ele...

a swarm, 6 x 80 = 480 cu ft!

The sub 225 appears to be a proprietary design that includes a 300Hz crossover.
https://manualzz.com/doc/28390566/duo-fact-sheet---avantgarde-acoustic

I would think your goal would be the capability of a subwoofer that has output and control well above 300Hz and not simply high output. 
Acresverde's experience and advise seems to be the only pertinent information to your goal since this is not just any speaker system.
Suggestion from first hand experience will save you from disappointment. 
Beware and good luck with your search.

Blending a single sub with the mains and getting the tone/impact you are looking for is much more difficult in a small room.  What is your room size?  Are you planning to use REW for measurements and or some form of DSP measurement and equalization?

Room size:

19’ 8” (wide)

32’ 8” (long)

9’ 6” (Tall)

No DSP planned at this point.

@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.

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.