This is an incorrect statement - speaker diameter does not dictate frequency reproduction characteristics
I tend to agree. I had an old set of Altec Lansings with dual 6.5 inch subs and it could play lower then many larger speakers. The cabinet design helped allot as well(very deep with dual ports).
I have doubts that many computer subs actually reach 40hz(not saying they can't) with much volume to be honest. I do not think I head heard much music reach that far down either.
Want to play with tones? MP3 is not quite ideal, but interesting none the lesshttp://www.testsounds.com/