The Ultra Tonic from James May Engineering is a soundboard transducer, like the K&K, but unlike the K&K, it adds BOTH onboard feedback resistance AND a volume control that does NOT alter the frequency response. To avoid that alteration, power (battery, phantom) is typically required, but James cleverly figured out how to do it and keep the whole system passive - no external power needed. It is a rather brilliant solution! The pickup is outstanding and I can easily recommend it.
The primary reason players will add a mic to a soundboard transducer pickup is to add some “air” to the signal path, but not have to rely entirely upon the mic’d signal (which is highly prone to feedback under higher performance volumes), and end up with what we perceive as a more authentic sound. It is a common pursuit, Whether it is worth the effort and expense, or not, lies in the ears of the beholder.