An MA109 landed on my doorstep today and of course I dropped everything so I could try it for a while until my wife dragged me away.
I'm just gonna throw out my impressions as they come to mind:
First, my existing rig is a Mesa Walkabout driving a Genz Benz Neox 112. Decent sound, but somehow lacking. I tend towards a more old school sound. I don't slap and play fingerstyle most of the time. My basses are a passive mongrel P bass with D'Addario Chromes, a passive Warmoth Jazz bass with Nordstrand Big Splits and D'Addario Nickel round wounds, a Kubicki ExFactor with D'Addario Chromes running in passive mode, and a Pedulla ThunderBolt 5 with Bartolini electronics and D'Addario Nickel round wounds. I've got Aguilar OBP-3, Nordstrand 3 band and Bart 3 band preamps each mounted in outboard boxes with full bypass footswitches.
Initially I'm playing in an average size bedroom.
Before I tried the MA109 I played through the GB 112 for a while to set sort of a baseline for what I was accustomed to.
After 45 minutes with the MA109, I have to say WOW! I hadn't heard any Big E cabs prior to this, so I wasn't sure what to expect.
When I first connected the 109 and started playing I thought it sounded nice, but not spectacular. As I kept playing, trying different basses, preamps and playing around with EQ I started to like it more and more.
The first thing I noticed was the low end clarity. As Mike said, "The 109 is warm and punchy with just enough tautness". A perfect description. The real eye opener came when I plugged in the Pedulla 5. The clarity on the B string was astounding! Actual pitch definition on the lowest notes! It seemed like B string tone matched the other strings better.
I can really hear the differences between basses and even the subtle differences between preamps. Granted, a lot of the subtleties will be buried in a mix, but the clarity still comes through.
Each bass retains it's character all the way up the neck. Tone and volume are much more consistent anywhere on the neck. Fewer peaks and valleys in the response?
I can tell the low end is opening up more as the speaker breaks in. It sounds even better on an Auralex Gramma pad. Cuts down on the room resonances as you'd expect.
I see what you mean about the Big E having a more immediate sound and less overhang on staccato notes.
I think the Walkabout is just enough amp for the speaker, but in a louder situation I'd want more power to preserve the headroom. As the level increases the Walkabout gets fairly compressed. I'd like to find an amp that retains some punch but still has the warmth of the Walkabout.
Two 109's would be awesome! I wonder if they'd sound better side by side vertical or stacked horizontal. Stacked vertically would be interesting, but a little strange too. You'd certainly be able to hear yourself.
Granted my testing is limited, but I did notice that the sound was much more consistent anywhere I stood. The upward firing tweeter is nice if you're forced to stand close to the rig.
With most speakers that have tweeter attenuators I've tended to run them no more than 1/3 up. With the GB I even ran it with the tweeter at max attenuation. I was a little concerned that the 109 didn't allow for that, but it's a total non-issue. I love the way the high end sounds. Totally seamless transition from low end to high end. I use a T-rex Bass Juice overdrive a little and it sounds better too.
The sound responds to changes in right hand position and technique and EQ changes much more than with the GB 112.
I'm looking forward to trying it in a band setting, though at this point, I can't say when that will be.
As someone else noted, I'm using the bridge pickups more than I had been. Much less irritating honk than with other cabs and a wider range of usable tones.
The form factor is great with a small amp. With the cab vertical it might be kinda weird with a big tube amp sitting on it, but at that point you'd want at least a couple of 109s.
Mike's construction is flawless. I never thought to ask about a handle or feet or corners, but there they are. The size, weight and form factor, and the handle location, make for an easy carry.
In the $575 price range (or more for that matter), I don't know of another speaker than can touch the MA109. An excellent value. I haven't played through anything that has sounded as good. Any misgivings I might have had about buying something I'd never heard before, especially a new model with no reputation or feedback, have vanished. I'm totally pleased with the purchase. I think Mike and Big E have a real winner with this one.
That's all I've got so far. I'm glad to answer any questions that I can, though my experience is limited at this point.
My overall impression of the MA109?
Playing through the 110 Flex has taken my bass playing satisfaction to a new level. Gotta agree with you 111% on the B string response. It's mind blowing and ear-gasmic to experience a cabinet this articulate across the spectrum. 
