FS does not exist when you put the driver in an enclosure, it is a free air measurement, there is no Fs to "play right through". In a ported box, the Fs (or resonant frequency) becomes the boxes tuning frequency or its Fb. In a sealed box, Fc is the systems resonant frequency.
Tuning below Fs is completely fine...it's done all the time. The only issues (or non-issues) I can think of would be the suspension limits & quicker unloading below tuning (less motor control to keep it in line). As long as you don't exceed Xmax to get to your desired tuning & have an appropriately set SSF, all should be fine.
This is from a 2010 discussion on FS over on DIYMOBILEAUDIO