I just got an e-mail from Tom Hazlett over at KA. I'd asked him about what drivers KA considered its top contenders as either woofers or tweeters. While suggesting that it's a "very subjective question," he said he's seen the TEC, CI and FK models used as woofers, the ED and FK models as tweeters, and combinations of TED + ED, CI + ED, CI + TWFK (triple) and TEC + TWFK (triple).
A couple of observations are in order. First, I think we've all seen the CI used as a woofer. That seems to be a no-brainer. Lots of pictures of customs, when blown up, show the CI on the outside, with its model number fairly visible. Second, it's interesting that the FK would be used as a woofer, in some designs, and as a tweeter in others. This is suggestive of the shapeshifting nature of BA's. There really aren't any obvious woofers or tweeters. It's what you do with these wide-range drivers that makes all the difference. Third, in dual-driver configurations, the ED may be a popular second driver, probably as a tweeter. Fourth, you'll notice that the TWFK is popular in triple-driver configurations. That's likely because the TWFK is a dual driver, the world's smallest.
When I piggy-backed my TWFK on my CI-22955, it was small enough to put the entire apparatus into a de-cored foamy. I was going to do this with the other driver when I had my soldering disaster. One thing I noticed was that there was ample room to piggy-back a second TWFK, to create a five-driver setup - and small enough to fit into (not onto) an eartip. With a 2.55mm minijack (let alone a two-prong wire connector), this would make for an amazing set of tiny shell-less earphones. Even without an "integrated passive crossover" or any filters to tweak the sound, I got amazing sound from my CI-22955 + TWFK combo. I found I had to position them right, to get a good seal, but once I did, it was an impressive combo.
Point of correction. I was able to wire up the CI-22955 for both ears (which worked wonderfully). I was not able to get the combo in the second ear because of the soldering disaster, forcing me to buy more drivers. What I did notice in the one ear (with the CI/TWFK combo) was a very cool, dynamic, sound.