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

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

You would think this  measurement would be on the Internet. I've looked at MartinLogan site and around the net, but can't find it. 

No it's not, as I found. You'll just have to do it yourself as I did, wish I saved it all those years ago.

Cheers George

Steve, here is Jo Fo's front speakers, as you can see his centre something his subs are not in the picture.
http://www.jonathanfoulkes.com/images/Animation%20FullFrontViewFlipBook.gif

Cheers George