The system Q of driver & cabinet has been used as an essential guide to a speaker's bass quality since the seminal work in modeling bass alignments using filter theory was done in the late 50s-early 70s by several people. The parameters identified are typically called the Thiele/Small parameters, Q itself is unitless and represents the damping factor (tendency to return to rest). There are several Q values (speaker electrical, speaker mechanical, box) in modeling a driver & enclosure, but speaker makers (as opposed to driver makers) will typically only use the Qtc of the bottom end, which includes the speaker and enclosure.
In general, a Q of .5 is critically damped, with no resonance & ideal transient response. A Q of .707 is considered maximally flat, with a 3 db rise at the system resonance. Higher Q values result in progressively more boost & poorer transient response. Note that I'm only talking about closed boxes here. All I can tell you about bass reflex is that transient response is always worse and the roll-off below resonance is 24 db/octave vs 12 for closed boxes. And don't get me started on those port plugs!
This is the measured response of the LS3/5A, according to Hi-Fi World. As speakers go, it doesn't look bad at all. The bass alignment can be seen in the steady climb to the left below 300 hz & peaking around 100.
