What makes me interested in the K4, is that although I know a lot of people hate multi pickup configurations because they can "phase",in my experience, especially in blending two separate Yamahiko pickups thru the 2 channels of the Felix, it is more about getting the blend ratio right. If you just bind the two together (Underwood, Yama,etc) then the ratio of one to the other is totally random and uncontrolled, but if you separate them and blend with a different ratio (based on your ear and bass etc) a wide range of tone and width and open/directness is available just in that narrow parameter.
The Wilson K4 has 4 pickups that can be individually adjusted in the bridge for that very purpose. Also, while the directions say to drill the holes 9mm from each string, there is no reason to be bound by that. Just the combination of simple adjustment for blend (twisting the pickups in the bridge) and the simplicity of the tube preamp/piezo thing makes it interesting. I played last night with the two Yamahiko pickups bridged and got a great sound. When I use them individually they have distinct characteristics (the G side has the even thump while the E side has the bass and treble...on my bass) and when I have blended them in the Felix they sound great 70/30 one way or the other depending.
THe K4 design could be a really effective way to balance the signal from each pickup (by changing the aim of the piezo element.) When I owned a K4 the last time it was way back before I even know about Impedance. The sound I got was thin and ugly. I think it deserves another go round...