The TQWT is among the best loudspeaker enclosures I know. It will let you hear music as music is meant to be heared: all the details are being passed to you at the speed of light and it will give you a up-front projection of the stage...Not every unit is good for this enclosure: units designed for the use in a horn won't do good at all... A good sounding unit to be used in this enclosure is the Fostex FE 208 Sigma.This unit is fast and tight sounding. If you want more "colour" the Philips 9710 is a good choice. Whenever which unit is used, be sure to tweak the enclosure, especially at the opening! A few centimeters difference can be responsible for a "lag"in the mids.... This speaker design is best used with tube-amplifiers, if you use one of the mentined untis and it sounds not right: I suggest heving a look at the rest of your installation:P 10 to 1 that there is something wrong there! I've also use a smaller Philips unit, the 3710 (5"). You'll find the balance in mids to highs somewhat better but it lacks the punch the bigger units have...
How to build.
The basic principle is by a mr. Voight. Fortunately he left lots of documentation about his design so we can easily adjust the imensions to our units.I experimented with lots of different enclosures and found that a small difference in size is not a important as you might think. Damping the enclosure can be done by using the most cheap carpet-felt available, as long as it has a foam back. This is glued in the whole enclosure apart from the baffle itself. The enclosure itself is made of MDF, be sure to put in braces where you can to optimize the rigidity! Don't put anything behind the unit, it wil choke the mids. I always leave the underside of the enclosure loose, to vay the vent at the bottom. This opening is room-dependent: try different sizes! Be sure to tighten the bottom before listening to it (see picture above), you'll be surprised in hearing the difference loose/fastened.I know for sue that this can be a cause of bad sound when a TQWT is taken home or to a friend and sounds terrible up there... Have fun building it!.