I have found the best combination of no fuss plug and play to be a soundboard transducer into a good pre amp. I have never heard an undersaddle transducer that sounded good to me, especially the ones put in under $500 electro- acoustics.
As a result of basic good fortune, I live very close to Teddy Randazzo of Dazzo pickup fame. I got a recommendation from a friend who heard Teddy play, and man can he ever, so after not liking the Baggs I had in my Martin, I had a Dazzo installed in another guitar. At the time Teddy recommended the RedEye pre that I've been very happy with. This pickup was a revelation. What it isn't is a mic, piezo combo system, which can be quite good but adds another layer of complexity.
Now let me clairify, most if not all acoustic amps have a high input impedance hardwired to the input or a switch to put it into the circuit. In this case, the amp has a pre in it and you don't need a DI or a pre. So you can match impedance with an onboard pre, a stand alone pre, or the pre in an amp. You can use the XLR output from an amp to send a signal to a PA, but this signal may not have a seperate output volume, so if you boost the volume at the amp, you boost the signal to the PA. You don't always want to do this.
Many outboard Pre amps have two outputs, one you send to the amp, the other to the PA. This way you can have your amp as a monitor you can adjust the volume on without infuriating the sound man. But if you want to use the effects on your amp, you need to use that amp's XLR to send signal to the PA. Just don't futz with volume once the PA mix is set.
So back to the outboard pre issue. A friend had Dazzos put into a Martin 000 16 I sold him years ago. It sounded fabulous. He skipped having an on board SunnAudio system put in because he playes through an acoustic amplifier. Well, two days later, he shows up at open mic and plugs into the PA. He sounded harsh and trebly and quacky, and I thought what the hell is wrong? I asked the host what he was using as a Direct box. The answer was nothing. Now I had not thought of this because I always used my RedEye, but then I realized that 90% of the open mic players with piezos were using nothing between the guitar and the PA. The Taylors always sound OK, because of the onboard pre.
Around this time Teddy had been working with a new company, SunnAudio, on some pre amp circuitry. It sounded better than my RedEye. They made m a box version of their Blender DI for the open mic. We had an A/B box and switched it in and out for folks. They were astounded at the difference. They now get patched through the SunnAudio DI as a matter of course. Onboard systems have a separate direct channel to the board. Now I'm told K&K states in their manual the need for a pre. It is retailers who do not educate their customers. Why? I think they think that their customers would not have pickups installed if the new they needed to buy a pre too.
As far as installation, it is not rocket science, but if the thought of drilling a hole in your $2000 guitar makes you queezy, or don't solder, leave it to a pro.
Now I think Dazzos sound way better than K&Ks, but some are reluctant to venture into new waters, but Teddy will consult with any tech regarding the right pickup match, and even send pictures. I have five sets of Dazzos for a reason. They sound very good, need little to no EQ, aren't horribly complex or expensive.
I can't say how thrilled I am with the SunnAudio products either, iceing on the cake to have a perfect match for the Dazzos, though they sound great with K&Ks too.