+1 I try to review a cab based on what I hear. Whether I like it or not should be relatively uninteresting to the general reader.
For the OP, the fEARfuls have incredible bass response. A very nice combination of 'going deep but not stupid deep' (i.e., no 'efficiency problems', or strange mechanical issues like with the Acme's IMO), and the 3015LF driver used in these cabs (and the 3012LF driver in the 12 cabs) has 'PA sub' mechanical specs, which means you can POUND on your B string if you want, and you won't fart these cabs out.
There is absolutely no low mid bump in these cabs, which is good for some and not for others.
The upper mid response, due to the crossover used in this particular version of a 15/6, and the very aggressive, bright, powerful 18Sound mid driver generates a LOT of energy in the upper mids (peaking around 2K to my ear) It is surely not 'flat'. Again, could be a positive or negative depending on what the player wants. For some, that is just the frequency area where you want to soften the response.
It would be key for me to have an amp with at least two semi-parametric mid controls, which would allow a player to 'bump the punch' if he so desired and also to soften that upper mid response to reduce 'clack, gank', etc.
Nice combination of big low end, bright top end, decent efficiency. If you are a DUB style player, forget about it... nothing comes close to it on the market. For other styles of playing, some will need quite a bit of EQ power to get you where you might want to go IMO.
For me, the relatively large size of the 15/6 (much larger than, for example, my Bergantino AE410 or 212), the limitation of amplification choices (the 8ohm impedance and big low end really needs an amp that can comfortably put out 500 watts or so at 8ohms to reach my typical gig volume), and for me, the additional EQ power it would take to get me where I want to me tonally, resulting in the cab not working as well as my current cabs. Pure IMO there.