The Tw6.1 is a mini-tower design using a 6-1/2" woofer with a 1" silk dome tweeter.  This was the first project I completed that really blew me away.  The key to success on this project was the oversized enclosure.  Optimally, the woofer would prefer being in an enclosure about half the size.  By giving the woofer extra volume and tuning the enclosure fairly low, we are able to get solid deep bass down to 34 Hz - incredible for a 6-1/2" woofer.  The other key to this system was a bass driver with good specs.  The unit in question is manufactured for Parts Express under the name Dayton Loudspeaker.  I'm told they are made in the USA.  The use a lightweight coated paper cone and a rubber surround.  They have a nice sized magnet and good power handling.  The fs is quite low, which allows for the deep bass extension.  The tweeter and terminal/crossover are the same as those used in the Bk5.1.  Teamed together, they produce a very smooth sound.  The enclosure is made from 3/4" particle board, screwed and glued together.  I used 2" PVC pipe for the port.  Ports can be purchased that have a more decorative look to them, but I wouldn't recommend going any smaller than 2".  I haven't experienced any of the dreaded "port whistling", but I would imagine a smaller port could produce some.

The sound reproduction of this system is quite pleasing.  They are probably best suited to rock and pop generes.  The midrange is slightly attenuated due to the larger enclosure.  The sound is quite detailed, and the system as a whole is quite clean.  The speakers offer good imaging.  The bass reproduction is particularly impressive.  Music with good deep bass will shake the walls.  Don't overdo it with poor quality bass, as overexcursion can ruin the woofers.