FR is a function of impedance. FR=f(Z). Higher the impedance @ resonance lower will be the output at that frequency. Typically for a woofer the resonance freq will be in the range of 40Hz to 60Hz. If the box loading offer a high impedance at resonance (sealed enclosure) then the output at that freq will be lower. Hence most sealed enclosure are not capable to provide very low bass unless its resonance of the driver is below 30Hz or lower. You still need a high Vas to generate substantial air movement and quite a higher power amplifier.Can you please explain why "clean" and "deep" is a function of the impedance?
Also, my response to rocksterraghu was for "Good speakers are able to deliver the whole frequency range at all volume levels"
In a bass-reflex enclosure you will have a low impedance at only that single particular freq (see its impedance curves) and will have 2 impedance peak at either side of its tuning freq. Hence most bass reflex gives a single freq boom bass.
Aperiodic & TL speakers have the advantage of quite a lower impedance at resonance (atleast 60% lower than the sealed counterpart). This ensures stable load for amplifier and lower electrical and acoustic impedance to the speaker system.