I am not sure why you guys think that this mounting reduces box load. The load the box sees is exactly the same no matter where the woofers are mounted. Box resonances or the potential for box resonances remain unchanged.
You make the assumption here that most of the energy that can excite box resonances comes from the changes in air pressure inside the box. It is very easy to show that this is not the case and much more energy comes from the direct connection of the driver basket to the box. Push-push done properly kills some (guess) 90% of the driver vibration thru active cancelation, and even more so at the lower frequencies that are more likely to excite the box in a negative manner..
If you want to control box resonances then brace it well and add No Rez.
It is still necessary to properly brace the box. And as to NoRez i have not had positive results using it.
That's well up into the heart of the mid-range. that's not a good place for a phase shift of chopping up the vocal region into dis-similar drivers.
There are compromises involved no matter where you XO. A lower XO is advantageous in that it keeps more of the music in the FR/midTweeter. Cross a bit higher and you can play louder. That is of particular importance with a small 3” FR used as midTweeter.
It is important that the helper driver has very good HF extension, the SF W14 reach up to something like 5k on-axis, the Alpair 12pw and Peerless 830870 we use in some other applications reach 10k, the other midbass we have used only goes to 1.5k but that speaker is also on a wide baffle and we have pushed the XO down to 160-180 Hz.
All of these designs were executed with an eye towards 1st order PLLXO, and that coupled with less than 1/4 wl XO point makes a phase coherent result easy.
The plate amps are not a problem if crossing below 200Hz. The size of passive crossover parts for the low crossover point are more of an issue if the order of the crossover gets high. Really large cap values or iron core inductors are much more harmful.
I have not yet tried every plate amp on the planet but of the many i have, the amps are of 2nd rate execution and the XOs not of the type we’d be inclined to use and usually only reach up to about 150 Hz.
I am a firm believer that an XO at about 200 Hz or lower is much better done actively, and all of our WAW have been designed with an eye to that. The passive XOs were designed to give a complete set of options for anyone purchasing a paid planset subscription.