Subwoofer matching

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.