Subwoofer matching

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.