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

Sorry Tim but you clearly have no experience with mating esl’s which are very fast (low mass) compared to a dynamic drivers (much higher mass), as there are big problems at the xover point to get them to seamlessly sound coherent and it is an art to get them to do so. A low FS for LF, and a low MMS for speed at the xover point is a good place to just to start, this is why the SV12 is an ideal choice.

Adding mass to a driver just to get it to go down further will create problems up higher exacerbating the problem where this xover point is most critical to have both esl and driver sounding/blending well to be coherent.

Over here a Monolith I (ported) sounds worse in the (above area) and is reflected in it’s used value around $3k.
A Monolith III (sealed) used fetches between $10k-$12K why would anyone want to port this and "maybe" get a worse sound.

BTW: Bean bag (pellet) fill (beans being the operative word) will give you an exact volume measurement of that weird shaped enclosure, as it takes everything inside the enclosure into account.
I hope your not thinking, leaving the fill in there as stuffing??? As that would be stupid. It’s a precise measuring tool, then to be taken out and put into a symmetrical container to work out the correct volume of the Monoliths weird shape enclosure volume.

Cheers George

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