fs means the frequency of resonace for a driver of free air its like the frequecy they move or vibrate or excur the most at free air thats wat that means.
like the btl have higher fs because its meant for spl like they will vibrate the most at its fs in free air.
and in spl competitions they only use higher frequensys like 45hz-65hhz a sub with an fs of 45hz will not even beat a sub with a fs of 16hz in a competition playin a 20hz tone. but if u take out the 20hz tone and you put a 50hz tone the sub with the 45hz will destroy that sub with 20hz fs.
alot of sub that have 38hz+ of fs its not meant to be a daily driver. but in most cases some subs that have good linear response and with good excursion will be able to handle a 15hz tone with alil audible distortion BUT a sub like the avalanche that had a 16hertz fs and you trow a 15hz tone and u compare the audible distortion from the the other sub the diference will be BIG in distortion if you compare both will be like the avalanche dont even have audible distortion and the other sub with 35+hz of fs will have definetly some good distortion at that tone.
a sub with higher fs will definetly be louder at higher frequensys than a sub with a lower fs.
some subs can play an octave lower than its fs. like the re audio xxx or avalanche since they got the xbl2 technology the have great linear excursion and they can play one octave lower than its fs.
subs will play the lows that are lower than its fs but they will sound like they are havin some problems or like they are out of control, but u can make a sub play another octave lower by building a ported box tuned very very low aorund 16hz.