Beware of the Black & White viewpoint that more LF extension is necessarily better....really it is more of a "gray" area. Extreme LF can be better if DONE CORRECTLY AND CAREFULLY using SPL measurements and careful Room design/treatments and a smidgen of EQ, however, in many cases extreme LF may actually be detrimental to the overall sound at LF and may even cloud some of the mid range detail.
See this interesting demonstration about hearing perception at Southhampton University, which should be considered when looking at the chart suggested by Tgrisham.
http://www.isvr.soton.ac.uk/SPCG/Tutorial/Tutorial/Tutorial_files/Web-hearing.htmIt shows how the pitch of an instrument can still be heard even if the loudspeaker lacks the ability to play the fundamental frequency!
The higher harmonics (or partials) allow one to hear the pitch of the played note.
The timbre of the note will, however, be affected by the relative magnitudes of the fundamental AND all the partials and how they decay.
Since room modes (also covered on this website) become severe at very low frequencies (easily up to 15 db boosts and nulls in most cases) then a flat speaker frequency response down to extreme LF may not necessarily be desirable. The room may alter the timbre of low notes more adversely than the loudspeaker response curve....no matter the cost/quality of the loudspeaker!
Therefore, with typical modest room sizes, a well designed speaker with less impressive LF extension can often outperform in LF timbre another speaker with greater LF extension.
There is actually quite a lot on this website that may interest audiogoners.