There's nothing fundamentally wrong with tuning below Fs. The statement "No driver can play below Fs" is from the bad old times before computer simulations and affordable measuring equipment and DSP-s, times when you had to rely on textbook box alignments and "correct" driver parameters and times when even 18" drivers had 5 mm or even less Xmax.
By textbook alignments no modern pro 18" with strong motors can go below 50 Hz, yet are used down to 30 Hz routinely. For example, textbook QB3 alignment for B&C 18DS115 is 22,75 liters box tuned to 57 Hz with F3 at 72,8 Hz - which of course is ridiculous.
By tuning below Fs you trade off max. SPL from about Fs to about 2*Fs, but gain significantly in bandwidth. You also need EQ to flatten the response. But as always, things get more complicated when you compare different drivers. E.g. around 20 Hz a high Xmax pro 18" (like B&C 18DS115) with an Fs of 30...40 Hz can have similar sensitivity and output capabilities, and significantly more output above 40 Hz, to a home theater 18" driver with Fs at 18...20 Hz.
As always, it's about what is more important to you and making appropriate compromises.
By textbook alignments no modern pro 18" with strong motors can go below 50 Hz, yet are used down to 30 Hz routinely. For example, textbook QB3 alignment for B&C 18DS115 is 22,75 liters box tuned to 57 Hz with F3 at 72,8 Hz - which of course is ridiculous.
By tuning below Fs you trade off max. SPL from about Fs to about 2*Fs, but gain significantly in bandwidth. You also need EQ to flatten the response. But as always, things get more complicated when you compare different drivers. E.g. around 20 Hz a high Xmax pro 18" (like B&C 18DS115) with an Fs of 30...40 Hz can have similar sensitivity and output capabilities, and significantly more output above 40 Hz, to a home theater 18" driver with Fs at 18...20 Hz.
As always, it's about what is more important to you and making appropriate compromises.
Well, as required as T/S has shown, i.e. the > Qts' is, the more below Fs the tuning required for flattest response. 
Qts' = Qts + any added series resistance: mh-audio.nl - Home
That said, I like EBS alignments for low Fs, Qts drivers since the roll off blends better with the room gain, which of course offsets some/all of the driver's declining efficiency with decreasing frequency, i.e. always trading efficiency for BW.
GM
Qts' = Qts + any added series resistance: mh-audio.nl - Home
That said, I like EBS alignments for low Fs, Qts drivers since the roll off blends better with the room gain, which of course offsets some/all of the driver's declining efficiency with decreasing frequency, i.e. always trading efficiency for BW.
GM
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