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

Thanks for the info Tim.   Finding that driver is the problem  :).  It seems they are very hard to come by so I'm trying to figure out what the next best thing is. 

just ran curves on the SV12 in 3 cu ft sealed.... It'll be 3db down @ about 31 hz with a qtc of .79

I can't be sure about the 3cu ft as I didn't save it, I think with a heavily stuffed ML box that came down to around 20-25hz and around .6 which was I was told by Neville Thiel (rip). Sliightly less efficient,  but that was fine as it was active xovered.

Cheers George

Congrats on your subwoofer purchase @stevepa   if you would, just go to the back of the box and get the woofer box measurements, we'll get very close on the cubic footage.   I'd hate to see you buy the wrong woofer.  Overall, I don't like over stuffing a box, (unless your box is too large) it actually raises QTC by limiting air flow, the woofer acts as though it is in a smaller cavity. A couple of other comments that were a bit off... Porting not keeping up with a sealed driver....Porting a driver in itself is not slower than a sealed driver... If optimally ported, speed should be fine...   Of course similar to a sealed box, if you seal a driver to get a qtc of say 1.1, it will get boomy on you, if you seal it for a qtc of .6, it will get lean... on a ported box, you can easily affect the curve with the port. Overall though, I would also want a sealed driver if I could find it. I prefer the rolloff and they don't go out of spec as easy over time.  Lastly a quote on driver cone mass slowing down the driver... Yes, if cone has less mass, it will be faster, but that would only be if 2 drivers had identical motor structure... with different motor structures, the cone mass alone cannot determine the speed of a driver. In fact, If you had a QTS of say .4 and you lowered the mass, qts would drop and sensitivity would rise, the driver would want a smaller box.... I have learned a ton from others on this site, so I don't want to come off rude or condemning,  but I do want you to see the facts.

I don’t like over stuffing a box, (unless your box is too large) it actually raises QTC by limiting air flow, the woofer acts as though it is in a smaller cavity.

Neville Thiel rip, said the opposite to me, it increases the apparent box size as seen by the woofer and lowers the Q.
Gale GS401 and Infinity 4.5’s were a classic cases of one of the fastest/tightest detailed bass at the time, heavily stuffed, the drivers had to be pushed down while tightening the mounting screws, sure they were probably a bit harder to drive. 

Cheers George

again,  stuffing in itself does not dictate speed. 
A lightly stuffed box is what a woofer is normally spec'd at, as you add stuffing, a medium stuffed box, does give the woofer the illusion of a larger box by slowing down the flow of air inside the box, the air has to filter through the stuffing.  When you over stuff the box, air flow cannot pass through and has the opposite effect.  Air can't pass through at all and the woofer reacts as though it is in a smaller box.....Maybe we have a simple question in terms about the amount of stuffing (what is lightly stuffed or heavily stuffed) As I stated earlier,  all things being equal,  I prefer a sealed box sub myself. Sorry,  but this is not opinion.  It works this way.
In a sealed box woofer, I have many time added mass to raise qts so that a woofer would go down better in sealed box.  The key there is "does the driver have enough motor & suspension to support the mass"  I hope this helps,  Tim