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

Yes, the driver ML commissioned Eminence to make is rubbish compared to the rest of the speaker, it has a one note bass, I’ve got all the T/S parameters on it, not good.

The ACI-SV12 ( Meniscus SW1284 ) I matched and replaced them with are miles better, for that box. You’ve got to find a used pair though. I could find much else that suited.

Heavily stuff the box so you get a lower Q (.6) to equal the speed of the esl panels, cross the SV-12’s over with 24db active at 130hz,
Keep using the ML’s own passive 18db xover to the ESL though, as it sound better than active. You’ll then have a speaker that comes close to the ML Neoliths.

Cheers George

Unfortunately when I got these they did not come with the passive xover.

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