Until someone compares the two Cap 2400's side-by-side, we won't know for sure, and I would still be interested in hearing what Jeff has to say on the subject. But, I think that Basshead is probably correct that you wouldn't be able to tell much, if any, difference between the two subwoofers.
There can be content below 15Hz in some movies, but we don't really hear frequencies below about 16Hz even if the frequencies are very loud and even if they are in isolation--such as with a test tone. Those lower frequencies just add more "weight" to the bass that we do hear, and they may create strong tactile sensations that we associate with very low-frequencies.
I do think that there is some value in having subwoofers that can go lower than 16Hz in order to provide that additional bass weight. So, I still see some real value in having subs that can go lower than the Cap 1400, or than the new Cap 118HT. But, how low is low enough is something that I really don't know how to determine.
It seems to me, that once you get under the low teens--13Hz to 14Hz--that any additional increases in bass weight will be pretty incremental, and may not be as noticeable to some of us to start with. So, if cost is an object, as it usually is for most of us, I honestly don't know that many of us would feel that the ULF version will be worth another $600 over the slightly smaller Cap 2400.
I wrote a post right after the new Cap 2400 was announced, remarking that Jeff loves to innovate and to fill what he sees as niches. But, as the history of the Captivators that was posted about a week ago demonstrates, the models evolve over time. A designer has to sort of feel his way along, trying different things to get to where he wants to go. And, sometimes he discovers where he wants to go through that experimentation. Some of that process might be at work with the new Cap 2400, and the improved Cap 118.
To me, the current Cap 2400 and the Cap 2400 ULF are a little too close together in performance. I'm not sure that both models will stay exactly as they are for very long. It seems to me that Jeff will do something else to separate them a little more. If I were guessing, I would say that he will do something else to the Cap 2400 ULF to try to get a little more separation from the Cap 2400. But, that is pure speculation on my part.
In any event, I think it is likely that if the Cap 2400 ULF, with a 10Hz port tune could satisfy you, then the smaller Cap 2400 will be able to satisfy you as well.