what did you can't understand?
What I'm posting is exactly what Vandersteen said.
The issue is to liberate/let it free the woofer of CW the frequencies in the low bass range.
That means that instead that the CW woofer plays/handled frequencies from around 35hz and up it will be free of the low bass frequencies and it will plays/handled frequencies from around 80hz-100hz and up in this way the IMD will goes lower in benefit of quality level of what you are listening today.
That's it, easy to do for you because you already has subwoofers that you can use to play the frequencies from around 80hz-100hz ( depending the choosed crossover frequency in the high/low pass filters. ) and below it.
If you can't understand it please let me know what you can't understand because it's not rocket science.
@rbrannen just posted about the Legacy Focus XD, please re-read his post.
This is the Legacy:
https://www.avsforum.com/threads/legacy-focus-xd-active-4-way-speakers-review-tremendous-towers.3030...that you can use full active with its internal amp or just active with low bass range. Please take a look the crossover frequency of the woofers and you can read that crossover frequency is 120hz ( active as a subwoofer. ).
I hope now you can understand the whole issue to improve the CW sound reproduction adding those subs in the rigth way.
R.