Advice on replacing 12 in woofer in Martin Logan Monolith III's

If you do raise the xover point to 300hz you get the problem that many speakers have of having two drivers doing the critical midrange. This is even much more accentuated with ESL’s.

This is why a Monolith with a 150hz xover sounds so much better in the critical midrange than a ML Summit with a 300hz xover. The reason Jon Fo went this way is because he uses 4 x Monoliths with external subs is for HT use with all that processing and that critical midrange xover in HT is not as big a problem than just hi-end stereo use.

Cheers George

"Sorry Tim but you clearly have no experience with mating esl’s"

But what I do have is a knowledge of how it works and why.  I do understand blending panels and a woofer...
I know that you guys don't know me from anyone, but when Sumo designed the Aria speaker, they did call me for advice and way back when,  The very company that you are speaking of Martin Logan did call me and ask me to come interview with them... which I declined. 
I'm old and I've been involved in speaker design since I was 19. So rather that turning this worst and telling you what you don't know, I will bow out....
@stevepa can easily private message me if he needs help. 

 Correction on the box size it goes back 6 inches before it starts to taper

That will make quite a difference.

Best to lay the Monolith on it's back take the driver out and stuffing, fill with bean bag styro bean fill, empty all into a known litre/gallons container (I use a 20lt contaner I had) to get the exact volume.
Then Google litre/gallon to cubic ft and use the calculator to find the exact cubic ft on the Monoliths bass enclosure size include bracing, esl power supply, wires ect.

Cheers George

You would think this  measurement would be on the Internet. I've looked at MartinLogan site and around the net, but can't find it.