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

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

I will still look for the SV12's but if I can't find them what driver would you guys recommend?  I'm just not as knowledgeable on driver specs.  Being that I 'm running a SVS sub,  I need a driver that is a good mid-bass.

@georgehifi     Hi George, it sounds like you and I are just mixing terms of what is lightly, medium or overstuffed, but overall are on the same page.
@stevepa     Steve,  get the outer dimensions of your woofer box on the back of your monoliths.  We can get very close to figuring its cu footage.  We need that to stop guessing. 
There were other woofers that did not go down as well, so if you are happy with that, using your New SVS sub, you'll be fine.  I would only worry about resale later. I'd want a driver that went down well. 
Get the dimensions and we'll go from there.

So the box is not square as George indicated, it is 25 inches wide and tapers to 19 inches at rear.  24 inches H and 11 inches deep

@stevepa    
Ok,  assuming a 1 inch front baffle and 3/4 inch everywhere else, you're looking at 2.469 cubic ft... so if we model on 2 1/2 cubic ft, we'll get accurate results.