Feastrex D5nf (4" cone) can achieve flat 50-20kHz with a planet-10 "Maiko" cabinet. It is very dynamic, ultra-detailed, musical and exciting. I found that 3W SS 12V car stereo chip was not enough power, and McIntosh MC402 (ss) worked fine but not as fun as tubes. Manley 100W PP tubes were the best sounding so far, while Cary 50W tube integrated sounded shrill and terrible. All are feedback designs. Many people power this driver with 5W SET no feedback amps successfully. It is ~92dB sensitivity, ~14 ohms impedance. Horn cabinet increases sensitivity. These are extremely expensive drivers (entry level $2600/pair) but the performance is enough that it can replace
any traditional multiway book shelf speaker with no regrets, except it won't be bookshelf sized anymore
Feastrex' larger, field coil versions can play lower and faster, with prices past $20k/pr. If you come to RMAF you can hear them play at The Lotus Group demo. The big challenge for me is box design. The drivers are so fast and clear, it is hard to make a box large enough to optimize the bass and still very low resonance to not pollute the direct sound of the driver.
Single drivers seem to prefer current source amplifier. You might enjoy reading this by Nelson Pass:
http://www.firstwatt.com/downloads/cs-amps-speakers.pdfRich
