[Joe L.]

The lower tuning frequency allows lower frequency sounds to be reproduced at a higher sound pressure level. This is an advantage for many of today's movie sound-tracks where low-frequency effects routinely occur between 10 and 20 Hz. Although most people cannot hear much below 20 Hz, it can be felt and adds a lot to the immersive movie experience. Generally, low tuning works with drivers that can move a lot of air. You need to model the driver, enclosure, and port in a loudspeaker-computer-aided-design program to see the overall frequency response.

As described in the paper I linked to earlier, output drops off rapidly (at 24 db/octave) below the tuning frequency in a ported enclosure. A sealed enclosure has a frequency response that drops off at a much slower rate. It still has significant output well below the -3db point of the system.

It is all a trade off. The higher tuning frequency of the DIY.com bookshelf speaker allows a much higher efficiency. Its smaller drivers would not be able to move enough air at lower frequencies to reproduce loud low bass (much under 55Hz) to keep up with the tweeter in any way. So... the designer could have elected to build padding into the crossover to make a speaker that could play lower frequencies, at lower SPL, but needing a higher power amplifier, or design a more efficient speaker that could not play as low.

Joe L.