Brace yourselves, this is going to be a long post!
The bad news is that I have been at home with a cold for the last few days. The worse news is that I have had more time to spend on AVS.
I have enjoyed the discussions on this thread, and have been coming closer to having some insight into why both Ray and Todd would have preferred the FV18 in Ray's room and the Cap 2400 ULF in Todd's room. To state it more plainly, neither of them liked the Cap 2400 at all in Ray's room, and I wanted to understand why.
I helped to advise Todd when he was selecting his subwoofers, and have communicated with Ray throughout his subwoofer journey, so they both have real credibility for me. Given the fact that I believe them, and believe that I would probably have reacted the same way, I have tried to give some thought to what could have produced that result? What was it that they heard, or at least experienced, that made the subs seem so different in the two rooms?
Ray has been explaining it in terms of a small closed room versus a large open room, with the Cap 2400 better suited to the larger open space at Todd's house, but not well suited to a small space. I think that he is sort of on the right track, but not in exactly the way he is thinking. First, let's try to exclude what we think it isn't. I think that the difference isn't low-frequency SPL. Both subs produce potentially significant amounts of low-frequency SPL, and my understanding is that Ray even backed-off the low-frequency adjust (LFA) on the Cap 2400 to the minimum setting. At that setting, it would definitely
have been producing less <20Hz SPL than the FV18, at a comparable volume level. So, the explanation can't be low-frequency SPL.
What about distortion, then? Well-made subwoofers should not be distorting at moderate volume levels, and I don't think that the two subwoofers were being pushed anywhere near their maximum levels. Plus, when they adjusted the LFA to the minimum setting, that would have taken a good deal of the low-frequency burden off of the Cap 2400, further reducing any possibility of distortion. But, they still didn't like the subwoofer in Ray's room, even then. It still sounded relatively heavy and indistinct compared to the FV18. So, it couldn't have been a difference in the amount of distortion, produced by the two subwoofers, that they were hearing.
Perhaps it was the inherent sound signature of the subwoofer that they didn't like. But, that can't be the explanation either. Both agree that the sound signature of the Cap 2400 is different from the FV18, but they both liked that sound signature in Todd's room--even with dual Cap 2400's. So, it can't be just the native sound signature, it still has to be something about the space. I went back to read all of the comments from the recent Houston GTG at Todd's house, and Todd described the Cap 2400 as "violent, pounding hard, more like a wrecking ball than a ballerina". Now, I think we're getting somewhere in trying to explain the difference they heard in the two rooms.
I'm not sure where I first picked-up the idea that below about 30Hz (and especially below 20Hz) we feel
low-frequencies as much as we actually hear them. I'm sure that I read it as I was researching stuff, but I have believed it for some time now. It can be very difficult to separate what we are actually hearing from the tactile sensations that accompany low-bass sounds. I believe that can be particularly true on a suspended wood floor, because the floor itself acts like a drum head, in that case, magnifying the tactile sensations with it's own vibrations.
But wait, both Ray and Todd are on suspended wood floors, so how can that have anything to do with the differences they experienced? I know just enough about construction to be dangerous, and I know that all suspended wood floors are not alike. First, the larger the floor area to be supported, the higher the likelihood that the structure--the floor joists--will be thicker and stronger, and perhaps also more tightly-spaced. That would reduce vibrations. Second, consider the surface area involved. I said earlier that a suspended wood floor can vibrate like a drum head, and it's actually a perfect analogy.
Think of a snare drum, with it's relatively small surface area, and it's tight surface tension, and think of how light a strike it takes to make it vibrate. Then think of a bass drum, with it's much larger surface area, and it's less rigid support, and think of how much harder a strike is required, by a much heavier drum stick, to make the bass drum head vibrate. Suspended wood floors are like that too.
I went back to look at the pictures of Todd's very nice house again. Not only is his floor area much larger than Ray's, but it is less rigidly supported. In fact, one side is not supported by a wall at all, but opens into a stair area. So, the drum analogy works perfectly. Not only is the floor of Todd's room more like a bass drum in surface area, but it is also less rigidly supported on the sides, and more thickly supported from below, and it simply won't vibrate as aggressively as Ray's floor will.
Now, if we put several ideas together, I think that we have a very plausible and likely explanation for the differences they experienced in the two rooms. First, is the fact that we can't really distinguish very well between what we hear, and what we feel <30, and even more so <20Hz. Second, is the fact that the Cap 2400 creates much more low-bass TR than the FV18 does. As far as I know, it creates more low-bass TR than any other brand of ported subwoofers. Todd's description of "violent" and "a wrecking ball" is part of that. It doesn't sound like a wrecking ball when it plays Mozart, or when it is providing bass support for a vocal solo. It only sounds violent when it is playing low-frequencies, and producing significant low-bass TR.
Third and finally, on Ray's snare drum floor, the Cap 2400 was like a bludgeon, producing low-bass TR that interfered with the ability to distinguish low-bass sounds. It wasn't distortion produced by the subwoofer. It was too much resonance produced by the floor. Put the same subwoofer back into Todd's bass drum floor, and those same hard drum strikes did not produce too much floor resonance, and they did not interfere with anyone's ability to distinguish bass sounds.
The <30Hz TR from the Cap 2400 would still have been abundant, when the LFA was reduced in Ray's room. The TR would just have been moved-up in frequency--more toward 30Hz. But, even with the LFA back to a normal level in Todd's room, the TR wouldn't have been so overwhelming. In fact, the bass drum floor, with it's large surface area, and less rigid surrounding walls (due to the open side), would have attenuated the low-bass TR for all of the subwoofers.
I think that this is the answer to the differences in what they experienced in the two rooms. Not all suspended wood floors behave in the same way, and Ray's suspended wood floor must amplify low-bass TR, through sympathetic resonance, much more than Todd's floor does. It's certainly logical that it would have to do that.
The reason that I think this analysis is helpful for Geo's thread is partly intellectual curiosity, for all of us, and partly practical. Concrete floors, sitting on soil or bedrock, just don't resonate much at all. So, virtually all of the TR that we feel has to come from the subwoofer itself, and not from the floor. Several of us with concrete floors have been observing what a difference that makes. But, I'm not sure that people with suspended wood floors, or those who don't really care as much about low-bass TR, realize just how
significant the difference can be.
I want Geo to get whatever subwoofers he wants, whether that is FV18's, or Cap 2400's, or something else. But, when we advise him, we may not just be talking about sound signature, or low-frequency SPL. Ray's and Todd's experience demonstrates that. We are also talking about what a factor low-bass TR can be for some people, and in some rooms, in the overall bass experience. So, the more that low-bass TR is a priority for someone, especially on a concrete slab, the more he may wish to emphasize that in his subwoofer selection.
I apologize for the long post, to anyone who bothered to read it all. (TLDR!) But, I thought this was an interesting problem to try to solve, and one with direct relevance to the OP's situation.