I know maybe three people here might be interested:
I'll recently replaced the Fe166En with the FE168EZ Sigma drivers in the BK-16 box. I've kept quiet for a while because I didn't like the results initially; until I realized I needed to put a 200uF cap in series, like how I did with the Fe166 setup before.
This time around, I settled on a 270uF cap which barely rolls off any bass - at least from a in-room listening position measurement. However, this cap did wonders for the FE168E Sigma. The Sigma is more mid-bassy, but this is partially because it is in the wrong BLH enclosure - one which was designed for the Fe166En. I strong suspect that the the Sigma does inherently have a more bassy or warm tonality though. But with the cap in place for a 1st over rolloff, the Sigma has a really clean, clear detailed sound. Without the cap, the Sigma sounded muddy and slow. Again, Daft Punk, Alicia Keys, and J-Pop do get played back on my system. Finally, the Sigmas are every bit as fast, snappy, and punchy as the BK-16.
In a nutshell, and I know some people have heard the Moth Cicada drivers, the Sigma when operating under good conditions is every bit as good as the Cicada. Keeping in mind that the Cicada drivers in the OB design were being rolled at 200Hz or so. Just as good, but different. The differences are not huge though. They actually sound much more alike than different. Both the Cicadas and the Sigmas have what I call a high level of "tonal purity". Sort of hard to describe or transmit what I mean, but they get the timbre just right.
The Cicada are more loose, sharper, but also less willing to stop bouncing around. The Sigmas are more controlled - less sharp on the attacks, but quicker to settle. The Cicadas extend more, but have more serious problems in the last octave. There is a massive spike at 10-11k, which actually isn't a bad spot to have it. Those of you who heard the OB setup at the last unofficial big non-HF Bay Area meet probably heard this sizzle. The Sigmas have a spike around 8k, but very minor. Some sibilance and grain from this if not careful with other components.
BTW, the Sigmas, unlike the Fe166en of the standard BK-16 kit do not require a super-tweeter. I think it does OK / acceptable though, at least for my old ears, of which I will make no bullshit claims of being able to perfectly hear way up to 22kHz like every person and his grandmother evidently can at other audio web forums. The Sigmas still slightly laid back, but not so laid back like the Fe166en. This is definitely welcome. A bit more bite on the trumpets and snares instead of being so forgiving.
The Sigmas would definitely be good candidates for midrange on up in an OB design. I'll try this in a few weeks. I think the Sigmas would also shine in ported box with sufficient volume + subs.
Pix below. Sigmas have thick cast frame. Surrounds are some sort of foam with funky pattern to prevent bad cone breakup (frequency spikes). Dust cap is woven into a phase plug with patterns which are probably there for some reason. LOL, note dynamat like material on back of FE166en frame. I put that there.
If I have time, I'll try to take 2' baffle measurements of both at 1/2 meter.
