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


I'm needing to replace the 12 inch woofers in my Martin Logan Monolith III's.  I am running a dBx 260 active crossover so I can control x-over points and output.   Any suggestions? 
stevepa
ACI made a passive sub kit called "ACI Titan" that used the SV12 many were sold, I believe through Madisound, maybe look for a used pair of these for sale.

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  
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