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

This guy had a pair for sale that no one picked up on, just "maybe" he still has them.

 http://www.highdefjunkies.com/showthread.php?t=14425&s=2374d39826a05548a2cfbdafd2d0c666&p=354307&viewfull=1#post354307

Cheers George

@stevepa
  If you can get the woofer cabinet dimensions so that we can get cubic ft or liters,  I'll take a look around... If I understand the Monolith III correctly, it is 89db sensitivity and the woofer is passive (not powered)....
Being 4 ohm,  you may find a good automotive woofer... the competition woofers are normally made for small boxes.  I suspect we'll find something decent.  Getting down to 28 hz in a small box will be the biggest issue. Get me box dimensions and we can all take a look around.

This was years ago, I did volume it using bean bag fill because it's an odd shape looking for the right driver, but I didn't log it down, don't hold me to it but I think it was 3 cubic feet.

Cheers George  

@georgehifi 
@stevepa
if it indeed 3 cu ft... this is expensive ($245 each), but ran curves F3 in QB3 alignment is 25hz 
Peerless  835017
If the box is a smaller, there are others that can work for less money.