I wish I believed that selecting a subwoofer, or a speaker, or a car, or a flavor of ice cream were as simple as just taking a poll of several people who have tried both models or flavors. Different rooms/objectives/tastes will inevitably produce different selections.
I believe that three FV18's would be outstanding for most people in a much larger room than Geo's, and I wouldn't hesitate to recommend them. I have already recommended them to several people. But, when I do, I am paying careful attention to what they have now, because that helps to establish a kind of baseline for what they are used to, and for what they may be looking for.
And, then I pay attention to what they are saying about the pluses and minuses of their existing subs. Are they looking for more clarity for music; more low-bass growl and TR for movies? What is it about their existing subs that isn't quite good enough, or strong enough? I just can't see this as a one-size-fits-all solution, where if we know someone's room size, we can tell him which subwoofer to buy.
I think that people may sometimes be a little hung-up on the performance for price issue. That is one of PSA's claims to fame, and I think it is a legitimate one. But, that's another one-size-fits all approach. Not everyone uses that kind of arithmetic approach to purchasing subwoofers, or other things for that matter. Sometimes, people just want what they want. And, someone who wishes that he could accommodate a pair of much more expensive Cap 4000 ULF's, in his room, doesn't seem like a good candidate for that kind of calculation.
I'm also not at all sure about the three is better than two approach. I think that three subs, long-term may very well be better than two, although that depends somewhat on the specific room. Personally, I really like having four subwoofers. But, I have been convinced for quite a while now, that we had better make sure that we are getting the right subs to start with, and then just adding additional multiples of those subs, as the circumstances require and the budget allows, rather than buying multiples of a different sub initially, because they are less expensive.
Depending to some extent on how I read the individual in question, I will usually recommend buying the most powerful single sub, or pair, that someone is honestly considering, or can afford. And then, he can always add to them when and if it is desirable to do so. I do that because we so often see people upgrading for MOAR. Does anyone know anyone on the forum who might do something like that?
Who's to say that if Geo buys a pair of Cap 2400's now, that he won't move to a larger place two years from now, and add another Cap 2400, or even a Cap 4000? I believe that Jeff deliberately made the port tunes on the various low-bass models the same to facilitate that kind of pairing. So, long-term, it may not come down to whether three is better than two, but whether the buyer started with the right two to begin with.
Ultimately, I don't think that anyone can predict what Geo and Ramona will like best, based on our own
experiences and preferences. We have to take our cues from what they have now, and from what they say they like and want more of. In an ideal world, Geo would be able to hear both finalists in his own room. And perhaps he will be willing to make that financial investment, as Ray did. Unfortunately, I don't think that the listening experience would be comparable if he auditioned both subs in a room with a suspended wood floor. I really do think that concrete makes that much difference with respect to low-bass TR.
In the absence of his ability to test them side-by-side, I'm not going to recommend based on cost, or on the basis of how three is better than two. I'm going to try to take my cues from what I believe are Geo's actual objectives and preferences and try to be supportive of those. I may guess wrong, but that's my goal. And, I am going to default to the best advice I know, which is to start with the most powerful subwoofer the buyer is considering. In this case, "powerful" would need to include both low-frequency SPL, and low-frequency TR, due to the concrete floor and due to Geo's personal preferences. And, when those two factors are taken, in conjunction, I don't really think that it's a close call.
But, that's simply one opinion based on how I read Geo, not on my own personal goals or opinions of the subwoofers. And, there is nothing whatsoever that says that I am reading Geo correctly, or making the right recommendation!
Edit: Geo, I saw your post above, where you mention cost, after I had already written this long post, but I don't think I will change anything. Just feel free to disregard anything which isn't applicable to your goals or preferences.