Hi Bassman,
I'm digging through the site here to find info on how best to setup my PMC AB1 monitors in my control room now that I'm rebuilding my (small) studio.
I've worked on flush mounted systems before and I love it, so I would love to have mine flush mounted as well. But I haven't been able to find anyone else who has exeprience with these particular monitors until now. And here you are!

I'm sure you love the AB1's as much as I do.
I hope it's ok to ask you a few questions specifically about those monitors flush mounted like you have them. I would absolutely love to drop by for a quick listen (and see your beautiful rooms of course), but Kentucky's quite a long way away from Denmark.
I'm curious to hear if you have experienced any problems? ...a need to tilt/filter the low end or anything?
Your wall construction seems pretty simple. Aren't there any problems with vibrations transferred to the front wall? Have you done anything to prevent that specifically?
In return, I can offer a little tip for you. I have always thought that the AB1's are awesome...yet they do have a little high mid/low treble sting that I would rather do without. So I tested a few different tweeters and I found one that is absolutely spot on for them. It has the same level of detail (I wouldn't compromise that) and character, but with that "sting" eliminated. Same price range as the original Vifa D27TG35 tweeter.
It's a ScanSpeak R2604/832000 tweeter.
Here's a photo for reference. It's large so I'm not attaching it here in the thread.
Note that it's 4Ω instead of the originals 6Ω. If you're bi-amping then just drop it in, but if not, then you'll need a resistor in series with the tweeter.
Also you'll need to "adjust" the hole behind the tweeter a bit + mount it with 5 (new) screws instead of the 4.
All this is well worth the effort in my opinion. My ears are more relaxed now that the slightly fatiguing sting is gone. Also, I think that when bi-amping, the woofers and tweeters are way more easily balanced to perfection.
If you experience the same stingy thing with your AB1's, then you should just go ahead and get these replacements. You'll love it.