The 168ez and the 166 can be used in a TQWT with added series resistance. The drawback is the lowered efficency. I have had the 168ez in a TQWT and even with series resistance the performance was not suited to my taste. The highs are almost painful and the only pratical answer was an EQ curve of sorts and i really dont like passive circuits in the signal path. I had the FE-166e in a BLH and the performance was exceptional, but i sold it when a friend made me an offer i could not refuse.The FE-167 has a higher Qts and will work in a pipe very well, it can even be used in a BR app but either both the BR and the pipe may still need some series resistance. ron