Since this issue is still alive, I would like to present a little different perspective. Chucky's compression graphs show that his very low-frequencies are actually higher
than his mid-bass frequencies, due to the combination of strong low-frequency response from his Cap 4000, and due to the significant low-frequency room gain he is getting in his room. Obviously, to anyone who hasn't been paying attention over the last couple of years, he really
likes loud low-bass sounds and tactile sensations.
But, as noted by an earlier poster, it isn't really necessary to "hear" 10Hz frequencies in equilibrium with other frequencies, anyway.
That is partly because we don't really hear 10Hz frequencies to start with. Instead, we hear some extra bass weight, and we feel ULF tactile sensations. And, there is some research that indicates that we can feel
low-frequency tactile sensations at significantly lower SPL's than we can hear them. Where ported subwoofers are concerned, that is partly because the very low-frequency TR isn't created solely by SPL, but is also the product of air moving through the ports. Air displacement, such as from an explosion, or thunder due to a lightning strike, is largely responsible for the tactile ULF sensation we feel. (This isn't port wind as a distinct physical phenomenon that I am talking about. We don't have to directly feel the wind from the ports to experience the ULF TR that air moving through the ports contributes.)
All of that is to say that I don't think there is any sort of fixed formula for determining how much SPL at 10Hz someone will need in order to hear sufficient low-bass weight, and to feel sufficient low-bass tactile sensations. Particularly with the big ported JTR subwoofers, I think that the tactile sensations would tend to be pretty strong anyway, with the right low-bass material, as the subwoofer volume was increased.
But, ultimately, I think it is going to be a YMMV issue as to how much low-bass SPL will be enough for a particular individual. That's why I think it's a very good idea to really push existing subwoofers, before upgrading, as a way to determine how much more low-bass SPL we are actually looking for.