Hi, The KSN1038 is the piezo tweeter Wayne uses in the smaller Pi speakers. I've used them in a few speakers and they work well. As for the audax, you'd have to run the T/S parameters through pialign or another speaker box program to tell if you got close on the box size. I didn't look up the FE166, but most of the Fostex line near that numerically are around 95dB/W while the Audax that are in the Parts Express catalog are around 90dB/W. What that means is that the woofer is going sound really quiet. The effeciency numbers for all the drivers have to be within maybe 1dB, or you'll have to pad down the mroe efficient driver(s). Chris