Martin Bluegrass guys love the K&K. I put the Fishman Matrix system in my Martin because I play in loud situations. The Achilles heel of the K&K is when you have to play too damn loud but otherwise it gives you a more natural sound that is true to the guitar. And as a bonus it is a lot cheaper than something like the Fishman Matrix.
Feedback is always a consideration, especially with a dreadnaught sized guitar. With any pickup in it, it is literally a "microphone", although a rather unusual one. My duo partner always plays a full bodied dreadnaught or jumbo guitar. He prefers the tone of the K&K pickups and has them in two of his three guitars. I suspect I'll have the 3rd one on my bench soon.
My Rainsong WS-3000 12 string has a Fishman Prefix Stereo Blender pickup in it and I do hugely prefer that to the LR Baggs that I pulled out of the OM-1000. The B string does not leap out at you for one thing.
It also has a blend control to add the built in condenser microphone although a little of that goes a long ways. Still, a nice option to have!
As I mentioned above, there is a notable improvement in the tone of the OM as an acoustic guitar from removing the undersaddle pickup and putting the saddle in direct contact with the bottom of the saddle slot.
I noticed this with the Martin all mahogany dreadnaught that I installed a K&K in for my friend, it had a Fishman system in it and once the full bone saddle was in place the guitar simply sounded more clear, defined and a small increase in volume Her husband is my duo partner and he noticed it as well.
I have a Fishman Loudbox Performer and it can get very loud without feedback problems if properly adjusted. It also has a "kickstand", easy to put it behind you (which reduces feedback) and turn it up as much as you need. The EQ section is a great design and there are 3 DI on the back. One for each channell (pre EQ) and one mix DI (post EQ). Depending on the gig, you could send the PA a dry signal or EQ'd with effects and mix the volumes of the guitars onstage, reducing the back and forth with the sound engineer. We use it as our entire PA system for many gigs we play, one of the clearest, fullest vocal tones I've ever had and surprisingly loud if needed (I usually play electric guitar for our duo gigs).
K&K and others do make preamps for the Pro Mini system, the pickup is part of the system but not the entire setup. Adding the preamp would probably help the bluegrass players immensely and take the price up a few noches. I don't see a reason for it considering the results I get from the Loudbox.