Yo Kenney! If you have Prefix Plus, it has a mono/stereo switch inside. Set it for stereo, and plug in with a 1/4" TRS plug with a splitter to two 1/4" TS plugs, and send them to the two DI inputs of any decent 2 channel preamp. I've had very good luck with a Joemeek twinQcs. The key is that the mic is too thin, and sucks, and the undersaddle piezo, though adding much needed mids and lows, comes with a fair amount of distortion, and it sucks!
However, if you start with a bunch of mic on one channel and feed in the pickup gradually, there's a sweet spot where it sounds like a wooden guitar mic'd up pretty. There's so little bleed, even from the internal mic, that you can sing along as you play, very, very, softly, just as a guide, then track the vocal later. One of the songs on the current project was an early track which I did that way 'cause I was out of practice, and didn't have the chops to play the damn song without singing it, at least a little. The track came out so pretty, we didn't have the heart to re-track it with mics. It was intended as a scratch track, but made the final cut.
The board's own Littledog listned to the track and said, " That was done DI? I'll have to send Fishman a letter of congratulations." He 's a critical listener, and hears lots of bad in music. It's the curse of the mixing engineer to listen to music, and zero in what's bad in it. I'm glad I'm not one.-Richie
P.S.- I don't know about the Fishman onstage, but I look forward to figuring out the voodoo of notch control.