Please understand that 'loud enough' is a misconception and not the whole picture. It's a matter of
impedance mismatch between the output of the passive piezo pickup and the input of an amplifier/PA system/recording-interface...
Please search the forum for 'impedance' as I have previously written tomes worth of explanation on this topic over the past 3 yrs here on UU, all of which is based upon my combined experience as a sound engineer, and my own personal use.
Volume level and tone problems are avoided by impedance matching, otherwise, even though it may be loud enough, you are effectively getting a 'high-pass filter' with a passive piezo, i.e., it's like setting an EQ with all the frequency bands from 400hz and below, ALL THE WAY DOWN, and even on a soprano uke, you are still going to be losing a bandwidth of about 325hz from 400hz and down, so no bass, very tinny and quacky sound, and very low signal-before-feedback, even with your amp volume cranked all the way up...
Please search - top right of every page on UU, and put in the word - impedance.
Having said that, I have installed MiSi Trio pickups in 3 ukes and like the sound and ease of use in that it does NOT use a battery, but instead a 'super-capacitor' that you recharge on 60 seconds and get 8-16 hrs of play time, and thus you save on the cost and hassle of the batteries...
if passive pickups are your purpose, you might also want to look at the JJB pickups which are similar to the K&K, but about half the price and have gotten good reviews. I plan to pick up at least one of these single-head and one double-head pickups for testing, and of course will be using them with an external preamp. see here:
http://jjb-electronics.com/our_products.html