Madisound has helped me and my friends in the past, they know what they are doing

https://www.madisoundspeakerstore.com/speaker-drivers-page-40/?p=catalog&mode=catalog&parent=2&pg=40&CatalogSetSortBy=id&gclid=Cj0KCQjwiZqhBhCJARIsACHHEH9-UOc_NxQIIhjozOuVni7fP3YnTbM9cbOFOVzuCgjpn1b7Nd1lsDIaAnG2EALw_wcB

Parts Express has people who also are helpful. They know more than you think.

https://www.parts-express.com/?utm_source=google&utm_medium=cpc&utm_campaign=6443420617&utm_content=78474321778&utm_term=parts%20express&gadid=377181913875&gclid=Cj0KCQjwiZqhBhCJARIsACHHEH92fIbfA6mMDCjcDL4RBHrnr-xeysAyA2f-SWcK1Z-8NDdfka3CqUsaAlhrEALw_wcB

The folks at Madison Sound are awesome. 

try some proac dealers local to you

a dear friend of mine had woofers on his response 3.8’s go bad and a longstanding san jose retail/repair shop who handles proac procured fresh replacement woofers for him, all is well now

you might need to do some legwork but proac has many dealers and i am sure some will help even though this isn’t a big money maker for them

the new proac distributor can be a little hit or miss when end users contact them, i have found

There don't seem to be any ProAc dealers anywhere on the East Coast; when I enter "New York" on their "Worldwide Distributors" page, it suggests a place in Toronto.

I did have a good talk with someone at Madisound; he was very helpful, but I'll need to do more complicated measurements for them to be able to help. Meanwhile, he suggested I get in touch with Speaker Repair of Colorado, so I've done that, and am waiting to hear back.

I really don't want to be in the position of literally throwing them out.

My friend bought his ProAcs here. Give John Rutan a call. I'm sure he will help you  out!

https://www.audioconnect.com/our-brands/

Do not throw them out.  ProAc Tablettes are very nice sounding speakers.  I would try to find the right replacement driver--not something off the shelf that might be a close, but not ideal, match for the original driver.  Locate the right driver, from or according to ProAc, and it should be easy to do the replacement yourself or have any competent dealer do the replacement.  It doesn't have to be ProAc doing the replacement if you have the right parts.

I did a quick search on Google and came across this:

a little proac info.

Discussion in 'Hi-Fi and General Audio' started by Lt Cdr Data, Dec 28, 2003.

 

Proac are a little mean on what they divulge, so I thought I would share a little of my hard earned knowledge, and give a kind of database of proac info for fans and curios
.
for some great proac klones check this out.

http://audioclone.free.fr/scanspeak.html

Older proacs are divided into 3 ranges,

1/the tablette
2/studio
3/ response

tablette 3, 50, 2000 inc. signatures use seas woofers and seas or scanspeak tweeters...2010 on signatures.

https://www.audio-forums.com/threads/a-little-proac-info.85593/

.

.

https://www.google.com/search?q=seas+woofers&source=hp&ei=sq0nZOD9MLqZptQPyIiD-Aw&iflsig=AOEireoAAAAAZCe7whMu2J2569iXq4MTShbcYU_nU0zR&ved=0ahUKEwjgnraR54f-AhW6jIkEHUjEAM8Q4dUDCA8&oq=seas+woofers&gs_lcp=Cgdnd3Mtd2l6EAwyCAgAEIoFEJECMgYIABAWEB4yBggAEBYQHjIICAAQigUQhgMyCAgAEIoFEIYDUABYAGCsPWgAcAB4AIABgwGIAYMBkgEDMC4xmAEAoAECoAEB&sclient=gws-wiz

.

Great info on this thread.  Nice research.

Just curious--after reading of these multiple accounts of woofers going bad I am a bit baffled (no pun intended).  Even driven to higher spl's, it seems odd to have woofers fail.  Thinking of how many vintage speakers voice coils have endured half a century of music making.

OP,

even if only one bad: always repair/replace both matching drivers, keep the working old one to use in the future while solving a new problem.

So if you can get a couple woofers shipped from Toronto, who cares if they come from Toronto?

@yogiboy is correct call audioconnect. They are in NJ and are a dealer. 
 

the ProAC site list dealers but in the  US distributor ‘is the sound org’.  I just bought ProAcs - waiting for them to arrive but the service has been great so far. 
 

https://www.soundorg.com

I would take out the dead woofer, look at the brand serial number, and order a replacement - if still available. Otherwise I will match the specs and order another similar pair.

OP here, finally following up on this. I've spent a lot of time talking to people, all of whom were very nice and generous with their time, but I seem to be at a dead end.

ProAc confirmed that the original woofer was the SEAS HO803-08, but that they don't have any spare parts for this. I've spoken with new and old SEAS dealers who also don't have a replacement (indeed, though ProAc was very specific about the model number, I can't find any evidence for it anywhere).

I spoke with the Sound Organization, the US distributor, who also don't have any replacements. They said that simply dropping in a similar-size woofer would be a bad idea, due to issues with how specifically the originals were designed. They recommended a few different repair or re-coning shops, none of whom were able to help. Madisound were unable to help, and I spoke with a few dealers (including the NJ and the Toronto ones) who also didn't have any ideas.

I think I've done all I can do, and as this particular line of inquiry is unpleasantly stressful for me, I think it's time to throw in the towel. I'd welcome advice about how to get rid of these—perhaps someone is more patient than I am, or needs just one perfect speaker, or needs parts.... Getting some money would be nice, or I could just give them away.

Thanks for sticking with this!

@the_jest 

If you want to send my the good driver, I'll run full curves and specs and then help you find a proper replacement. I am in the St. Louis area. 

one idea not mentioned is Millersound who repairs drivers, in PA

http://www.millersound.net/contact-bill-legall-millersound.html

ProAc not listed, but SEAS is.

your driver is

unique face frame and rear basket (you have)

unique bullet (you have)

cone (is your cone good?

voice coil (perhaps blown). IF it is the voice coil, perhaps SEAS used same size voice coil in other drivers, i.e. Millersound might have the voice coil.

btw: to prove it is not the crossover: have you tried temporarily moving the good driver to the bad woofer's location?

Seas did indeed use the same woofer. I have used them a few times. The Proac driver is an MP14 series, they customized that series with a phase plug for Proac. I have only seen the phase plug version made for Proac. Look at an MP14RC (H0688) or MP14RCY. These were both listed as Midbass drivers

Well, I hadn't originally contacted Millersound because their website strongly discourages you from getting in touch about brands that are not listed, but, as pointed out above, they do list SEAS. So I emailed, and they in fact think that they would be able to repair these.

I'll have to figure out how to remove the woofers, which scares the hell out of me, but I'd be thrilled if I could in fact get the whole thing working again.

I confess to being exhausted by this project, but....

Will update. Thanks for the suggestions!

I think this will be the final update.

I did get in touch with Millersound, and Bill LeGall was extremely patient with and helpful to me. He doesn't normally do this kind of work, but agreed to take a look, and in a short time he was able to completely rebuild the woofers (both of them, though only one was dead (and it was indeed dead, it wasn't the crossover or anything else)). I was able to remove them and reattach them, which is rather more tech work than I normally do (Bill was kind enough to leave stripped wires and send me wire nuts, obviating the need for me to solder them, which would have taken another year off my life).

I'm still breaking them in, but the two important points are that (1) they work and (2) they sound pretty awesome already.

Thanks again to everyone for their help. This was more work than I'd wanted at the start of this process, but I'm glad to have these speakers working again.

Congratulations on achieving what you aimed for. Now have fun!