A general rule of thumb, for what that's worth, is that you should not tune a TL lower than 0.707xFs.
Bob
Theoretically 
you can tune the box to the lowest freq ,but this lends to have a big air column moved by the membrane ...the lowest the freq , the longer the column . So the cone is loaded at the lowest frequencies because the mass of air ,which is a fluid , can vibrate together with it ,and raising the frequency it looses the spring effect . So ,depending primarily on the driver's
characteristics ( Qms )and the geometry (folds) of the labirinth and the acoustic resistance (stuffing),the mass of air contained inside effectively lowers driver's resonance freq , but .... air is rather a lazy fluid ,and after a transient reproduced by the woofer ,before the fundamental exits the terminus
, time passes ...and often there's no perfect design that can overcome this issue .
you can tune the box to the lowest freq ,but this lends to have a big air column moved by the membrane ...the lowest the freq , the longer the column . So the cone is loaded at the lowest frequencies because the mass of air ,which is a fluid , can vibrate together with it ,and raising the frequency it looses the spring effect . So ,depending primarily on the driver'scharacteristics ( Qms )and the geometry (folds) of the labirinth and the acoustic resistance (stuffing),the mass of air contained inside effectively lowers driver's resonance freq , but ....
air is rather a lazy fluid ,and after a transient reproduced by the woofer ,before the fundamental exits the terminus, time passes ...and often there's no perfect design that can overcome this issue .
Me ?
I haven't 'played' with TL as seen on early '70 (60 ? ) designs. I prefer it tapered and with the driver not placed near the closed end ,i.e.no surfaces in the vicinity of the cone ...You know how DIY enthusiasts take their approach to the project ,they want to do everything perfect ...despite the final judgement may be compromised by some self-ego...
Surely the bass is there ,I just don't know if at some listening levels (which may be even not too loud ) the de-correlation of emission from the woofer and the terminus makes happen some alterations like phase issues -ripple or whatever .
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