I've heard many fine examples of compression tweeters and midrange drivers, some of which employ titanium diaphragms (my speaker came with a modern titanium diaphragm compression midrange driver with alnico magnets, but, I switched that driver out for a much older compression driver that has a bakelite diaphragm. This midrange driver, a Western Electric 713b, is one of my favorite compression drivers. I also like the Western Electric 555 and the G.I.P. Laboratories' clone of the 555, but these add the complication of requiring a power supply for the fieldcoil magnets.
Perhaps the best sounding compression tweeter I've heard is the G.I.P. 9501; it utilizes, I believe, an aluminum diaphragm, and costs a staggering $60,000 for a pair of tweeters (not including a power supply for the fieldcoil magnets).
