I am still interested in Ken's take on it. Particularly since he went composite. I'm still fighting the last of the 15/6 vs TC115 issues out in my head. I'm going with Mike, either way. TC115 form and 4ohm and port tuning has me thinking good things. However, fEARful has more exposure and no proprietary components, so repair issues seem easier down the road, and the modularity for expansion is attractive. I have no doubt they both sound amazing, though in slightly different ways.
I'll post up in a while when the cab is broken in, and I have some gigs under my belt.
One thing I will say at this point is that the Thunderchild voicing is quite different from the 15/6 or 12/6 voicing. I've spent much time with a variety of 15/6 and 12/6 cabs, and of course, have many gigs on my TC112.
Since Duke's design goal with his 15 is to basically make a 'louder Thunderchild 12', I feel like I have a pretty good handle on the difference (edit: I'm sure the slightly lower crossover point and more heavy duty compression driver impact the top end of the 15 version of Duke's cab a bit versus the 12, but most reviews that I've read describe the general tone very smilarly between the two models).
Tom's comment of the 12/6 and 15/6 top end being a bit 'clinical' is right on. Duke's cabs sound like REALLY good bass guitar speakers (Roger Baer's cab is the same way). Plug just about any head into them, and you are good to go. No muss and fuss on fancy EQ, high passing, high power at 8ohms, etc., etc. (IMO and IME) The 15/6 and 12/6 executions using the 18Sound mid driver are more 'PA cab' sounding. Very even upper mids, even in that area around 1 or 2K that can result in some trouble with many heads and basses (i.e., harshness, gank, brittleness). You really need a head like the Streamliner that has a naturally sweet voicing up there, or a lot of EQ power (like that Carvin head that many fEARful guys like) to make the top end of these cabs sound 'musical' IMO.
The low end is quite a bit more extended and 'big' sounding with the 15/6 and 12/6 cabs versus the tighter voicing of Duke's cabs also.
So, quite different in size, voicing, everything. If you don't need to shake the ashtrays in the room, it is hard not to like the beautiful, 'custom' voicing of the Thunderchild cabs, their small size, and the nice voicing options with the port plugging and that wonderful treble roll-off switch. If you are playing all out rock or whatever, and are looking for the maximum wump in the low end, and have the skill and motivation to work with that rather clinical (IMO) upper mid response, the 15/6 is quite impressive.
Interestingly, the Streamliner just SINGS through the 15/6, where it sounds a bit 'flubby and dull' through the Thunderchild to me. That warm, smooth, creamy voicing is perfect to make the 15/6 sounds 'musical' (for lack of a better term), but is a bit too much of a good thing with my Thunderchild. The low end of the Streamliner is also perfect for the 15/6 cab.... big, but VERY controlled with the very nicely designed hi pass filtering that the Genz engineers use. I do find myself pushing the preamp gain control quite a bit more with the 15/6 versus my Bergs or whatever, just again to 'warm up' that upper mid response that can get a bit 'ice picky' if you aren't careful. Dialing in just a touch of hair with the Steamliner really sounds nice.
Anyway, blah, blah, blah.... they all sound good in their own way, and Tom's posts above pretty much nail it.