Mike and I have been going back and forth on cab design/sound for a couple years now. I'm always interested in new designs that can get close to the sound and feel that is in my head (they are companions to the voices up there). I had a 45/110, and both steubig and I found something really interesting going on with that cab (especially the 45), particularly when you let notes and certain intervals decay. But is was way too thick in the low-mids for my taste, at least at that point in time. I also had a TC212 composite (Duke design, Mike built the cabs) but that was overkill for what I was looking for.
After the 45/110 experiment Mike and I talked a couple of times. He said I was part of the impetus for the Flex design as he likes to have good support in the low mids, so his cabs went that way sonically. The Flex was his take on building something that could be a little more lean there, and have a bit more edge in the upper mids. For me though the Flex is too heavy - I'm at about 100 gigs a year, schlep my own stuff, and have chronic back/neck things I need to worry about so I won't own anything over 35 lbs. When I saw the MA109 I was interested, especially due to the form factor. When I saw the MAS109, I gave Mike a call to see what he thought. He said he thought I'd really dig it, as it has low extension (which I was looking to augment in the H1203), but not nearly the thickness in the low-mids. Given that I was thinking of running it underneath the H1203 on bigger gigs, he suggested a low-pass filter on one of the input jacks so I could run it like a subwoofer (well, more of a woofer) with the H1203.
The H1203 really is a magic cabinet. It is small, light, loud, and has a sound that I really like. I vastly prefer a 3" paper cone driver over a traditional tweeter/compression horn, especially since I like a bit of grit in the upper mids. That's one reason I gigged Roger's ML112 cab for a year or two. Mike has been using the 3" for some time, and Duke and Roger do, and now Genzler is as well. The only criticism that I can level at the H1203 is that it doesn't reach that low. But you're running into the realities of physics - you can't have everything, and a small/light box is only going to go so low. That said, Duke's design pushes it further than any other small cab I've tried, and I've tried a lot. The fact that I can cover most of my gigs with a single H1203 and no FOH says something.
The MAS109 is a different profile. The upper mid edge is similar, since it is using 3" paper cones like the H1203. The low end profile is different though - 10" speakers just sound different than 12" speakers. When steubig and I were comparing, the adjectives that come to mind are that the 12" is more "open" than the 10", while the 10" is more "focused" than the 12". That's one reason the pair is phenomenal. Plus the MAS109 extends low and does so without a lot of boom or bloat.
Just because I'm CDO (that's OCD but in alphabetical order), I'd dig the H1203 in a box that fit perfectly on top of the MAS109. Or if I really wanted to get greedy, I'd want an MAS109 that was about 14" deep with a recessed speakon jack (I'm on tight quarters a lot), and then a 12/3 module to sit on top. That said, I'm seriously considering another MAS109 but with the Claire tweeter, and then I'd run that as a single cab for smaller gigs, then run the one I have underneath for bigger gigs.
I've been getting my kicks for the last couple of years (and feeling quite spoiled) with a Reeves Custom 225 and a Shaw B-150. Satisfied grins ensue every time I fire one up with a Flex 110. 
