Lots of folks like the K&K transducers ...I have steadily gone on record as not liking their tone. That said, the LR Baggs I-Beam is a decent under-bridge transducer. They make a passive version, so it's not only less to install, but no fuss with battery changes; and even better a nylon-string-specific version that is presumably tweaked for this application. I have one on a solid-topped cedar/lam body spanish guit and it sounds decent plugged in direct, and quite good through a DI which has a tone-shaping abilities. FWIW, I personally think it's better than a K&K. But there are likely newer versions of passive transducers out there ...just make sure they are voiced for nylons as that will likely get you a more natural amplified tone.
Edward