Quote Originally Posted by tin ear
Was that motivated by the listener or by the industry? I wonder... If a speaker won't faithfully (or reasonably anyhow) reproduce the full range of human hearing, then you need to buy more speakers to fill in the gaps, right?
More like improvements to the technology and cyclical trends. Ported speakers require a precise match between the driver characteristics, the cabinet volume, and the size of the vent. Once the Thiele/Small parameters were standardized starting around the mid-70s, manufacturers and hobbyists could accurately and inexpensively predict a ported speaker's tonal characteristics with different cabinet/vent combinations. Less trial-and-error and prototyping needed to get a ported design to sound right. The advantages of a ported design are their greater efficiency and more linear bass response down to the tuned port frequency.

Acoustic suspension designs are generally more forgiving, which is why beginner DIY hobbyists will often go with a sealed speaker for their first project. Subjectively, some listeners note that sealed speakers have a tighter and more natural sounding bass because they have a quicker transient response and a more gradual dropoff at the low end. However, the dropoff in the bass also starts higher up in the frequency range compared to a ported speaker. I think a big part of the ported speaker's popularity is because the lower efficiency and bass dropoff that begins sooner put the sealed speakers at a disadvantage in a dealer showroom comparison. A ported speaker will likely sound louder, with more bass. A sealed speaker can go deeper, but it won't go louder. A lot of other factors are in play, but those are the general differences.

As far as the full range goes, a lot of speaker design comes down to how far into that lower octave you want to go. n order for a main speaker to cover most of the audible range, they require very large surface area and/or large cabinets. It's difficult to get keep cabinets that large from resonating, which is why the full range speakers cost so much more. Subwoofers are a very cost-effective way of going into the lower octave because it only has to cover a limited range, only requires a single cabinet, and doesn't have to worry about how the bass response curve will affect the higher frequencies. More speakers won't give you more bass -- more capable speakers or a subwoofer will.