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

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.