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

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 

With a lot of advice and help from Tim (Timlub) I purchased a Dayton 12 inch driver. Since I'm running a dbx 260 active xover, I was able to to dial in the xover points and the slope,  it sounds amazing.  Also I added a SVS sub, which really brought out the lows.  Still have some room EQ I need to deal with, but overall it sounds awesome. Thanks Tim!  

Thanks for the kind words Steve. I'm glad that it worked out for you.