I don't think databass has tested the F25, have they? Since it is a sealed model, I would expect there to be a more gradual rolloff than the FV15, so the output at 10Hz might be closer than you think. That being said, the lower you go below 15hz the less and less content will be there (okay, I know 20yrs ago that argument was made against anything below 40hz....) and once we get below 20hz there is really nothing to be heard, only felt. So unless we are talking about extremely high levels and enough woofage to create a pressure vessel in you room, there is a good chance that extra 2hz would never be audible. I think you see where I'm going with this...go with the single F25 for now. There is good chance it might perform so well you won't even want to spend the extra cash on the second. (although I am big proponent of multiple subs) As we discussed in the other thread, the Rythmik is going to give you a better bang for you buck, even if you could get the Sub 2 at the same price as the Rythmik.