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

If I can't find ACI-SV12's,  what else should I look for?

They were too expensive to make that’s why they were stopped, but many were sold, dig deep for used ones.

The SV-12 were only one I found that went low enough and up 130hz, also known as the Meniscus SW1284. There were some that had as good LF performance but they weren’t to good up at 130hz, meaning they had to be cut off lower at 100hz and the ESL isn;t good down that far.
Here are the T/S specs which ended being perfect for the Monoliths sealed enclosure.
Especially if you used the SV-12 Third Order Sealed System which is just a 2500uf non-polarised cap in series with the + line, takes the +2db bump out before it rolls off at 25hz and takes it a little lower to 20hz, very tight and flat and protected against amps going dc.

ACI SV-12 T/S
Nominal Diameter: 12",
. Impedance: 4 ohms,
. Range: 15-500Hz
. RMS Power: 350 watts,
. System Power: 500 watts,
. Sensitivity: 90 db
· Magnet mass: 1930 grams,
. BL: 8.9 Tm,
. Fs: 17.5 Hz
· Qms: 8.35,
. Qes: .41,
. Qts: .395
· Vas: 250 liters,
. Cms: 1041.67,
. Mms: 99 grams,
. Mmd: 93 grams
· Xmax (linear one way): 12.87 mm,
. Voice Coil Diameter: 2"
· Sd: .0460m2
· Voice Coil Height: 31.75 mm,
. Air Gap Height: 9.652 mm
· Voice Coil Inductance: 1.45 mh,
. Voice Coil Resistance: 2.97 ohms
· 12 3/8" overall width,
. hole size 11 1/8" Flange depth 3/8" (1/2" with gasket)
· Overall depth from front of flange to rear of magnets: 6"
· Driver displacement: 2 liters, (130 cu.in.)
· Heavy duty, gold-plated, high-pressure solder less terminals

http://cdn.avsforum.com/f/ff/ff5db2a3_vbattach139511.jpeg

http://www.madisound.com/store/manuals/SV12Specs.pdf

Cheers George

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

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.