47K ohms is unlikely to sound good with K&K. People who try the resistor trick are trying to lower a 10 M impedance down to K&K's claimed 1M. In reality, anything over 500K or so will work fine with K&K (including 10M or even 20M) in my experience. However, below 500K you will start hearing low end roll off. 47K is likely to be pretty thin sounding. This amp would work fine with active pickups that have low impedance outputs, it's unlikely to sound good with a passive piezo. Of course, depending on what you like, you never know. Some people's objection to K&Ks is too much bass, and 47K would definitely solve that!
BTW, here's an impedance test I did a while back with various impedances. I don't have a 47K example, but I have 330K and 10K. 47K would be in-between, closer to the 10K. (10K is the 3rd sample in the list)
impedance sequence: 1M, 10M, 10K, 1M, 20M, 330K
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