Save this chart someplace handy ...
Physics of Music - Notes
When you said the Delta was rated at 65 Hertz, I confess to being a bit confused.
Did you mean that the Fs ( Resonant Frequency) was 65? The Eminence website lists the Fs of that driver at 41 hz, so pardon me if I seem confused.
To address your other question, the human brain is a wonderful thing. When your ear hears the harmonics of a LF
note, the brain assumes the note is being sounded. Your particular speaker may not reproduce 41 hertz well, but you will hear it regardless. Actually, many bass speakers have F3 values quite a bit higher than 41 Hertz. F3 is the point on the left shoulder of the SPL vs Frequency curve where the response decreases by 50% (3db). Other speaker and cabinet parameters such as box volume, vents and ports can affect these values.
( I know the description is over simplifed ...)
I hope that helps.
There is a very good FAQ by Joris on the site on speaker and electronics design you may find quite helpful in exploring speaker questions and other related issues.
here.