Fs is simply the resonance of the parts in a free air environment, in all honesty it is pretty much a worthless parameter.
It does not indicate the frequency response of a sub (lower Fs is not going to automatically "hit lower" contrary to popular belief) and Higher Fs does not mean that a sub will not "hit the low notes". A speaker simply does what you tell it to do in the environment that you put it in. Putting a High Fs subwoofer in a box tuned to 33Hz does not mean it is not going to play the frequency range from the Fs to tuning..and totally miss it. That is not how it works and is totally urban myth. It will do whatever you tell it to do, including destroy itself. Simply because it does not know any better.
Qts is the "overall Q" of the speaker, and is something that dictates motor force and what kind of enclosure a speaker should be in.
Low Qts woofers (say .35 and down) should be in a ported alignment only.
Mid Qts woofers (say .36 to . 45 or so) works well in a sealed or ported alignment.
High Qts woofers (say .46 and higher arbitrarily) works best in a sealed alignment. Again, these figures are arbitrary and what I feel/think is 'optimal' for a musical scenario and daily driving. This is not a law, it is simply what I think and is my opinion.
However, you can stick a high qts woofer in a ported enclosure..and get one heck of a peak out of it...and make it work great for SPL .
Qts is indicative of motor force as well. (Bl)
Lower Qts will allow more motor force, Higher QTS will not allow it.
Hope this helps.