Why is a speaker cabinet with a transmission line better than a cabinet with a simple port?

The definition of a "transmission line" differs depending on who you ask or what you read.  A simple port by some definitions is a type of transmission line.  But we distinguish our designs from simple ported designs because we don't just cut a hole in the cabinet and insert a straight plastic tube.
Instead, we build a channel in every cabinet that goes inside the cabinet and up at least a portion of the back wall with a length that is equal to a minimum 1/12 the desired wave length.  In same cases this transmission line wraps around inside the cabinet.  The difference is obvious in the improved bass performance (both depth and control) of our speakers.
Our longest transmission line, relative to the cabinet size, is seen in our Level TWO Powered Subwoofer.  Follow this link and then click on the tab labeled, "More Images": http://www.htd.com/Products/powered-subwoofers/Level-TWO-Powered-Subwoofer
We do not publish all of our mechanical drawings because we consider this proprietary, but we do show this cut-away as a good example of a key difference in our cabinet designs.  This feature is only seen in much more expensive brands.  Our lower price is simply the result of our business model of selling our products only direct to consumers.
It is also significant to note that all our cabinet-style speakers are "front ported".  This design allows you to place your speakers close to, or even directly against, a back wall without negatively impacting bass peformance. While a front port design costs a little more to manufacture, its importance becomes apparent (especially in home theater setups) when determining the location of your speaker.