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

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.

I have many time added mass to raise qts so that a woofer would go down better in sealed box.

Adding mass to the cone does lower the FS, but you didn't mention it also raises the distortion (slows down it's reaction time) at the higher end of the woofers capabilities.
You give to one end but take away from the other, there's no free lunch.

Because it's such an odd trapezoidal type of shape, I'm positive when I bean bagged filled the bass chamber to do a proper volume check, it was more than 2.5cu ft.

Cheers George