What Do You Think . . . and How Does It Work?

Kenjit, I’d say millercarbon is correct and the effects of pressure on the inner walls of a tube (incidentally, specifically engineered to uniformly hold pressure) is a great idea. Cabinet resonance would be low using this material and geometry, especially if you used schedule 80. Talk about inexpensive DIY!

If designing a tube shaped cabinet, it would be logical to use at least an inch or two larger radius (inner dimension of the tube) than the ports on the driver basket to allow the driver to breathe correctly, with a beveled edge on the baffle to allow unrestricted airflow from the back pressure of the driver.

The bazooka style used in car audio worked well.

Kenjit, explain to me why it wouldn’t work??
(Oh and I don’t specifically think it’s not the material so much as the geometry of a tube that makes it such a good idea).

Kenjit, explain to me why it wouldn’t work??

of course it would work. It just wouldnt be state of the art. Look at all the other high end speaker companies and the great lengths they go to with their cabinets. There are no PVC speakers out there among them. Only PVC speakers out there are the cheap ones on amazon. 

Kenjit, um... concrete pipe?
And yeah, certainly competitive with other devices claimed as state of the art. A cabinet's audible colouration (coloration) has a significant signature to the overall sound, a tube will do well in such a situation as the tube itself lends to withstand pressure from the driver.

Let's face facts though, best engineered for sound quality, isn't necessarily aesthetically the most pleasant.

kenjit,

from an engineering standpoint a tubular shape is inherently the most rigid and deflects the least under pressure than a box shape.  
that's why pressure vessels are cylindrical- e.g. pressurized gas, liquid nitrogen storage dewars, compressed air cylinders and HVAC boilers.  
as a speaker enclosure this translates into ultra low resonance and coloration.  Magico in fact sees the value in ultra rigid enclosures by making theirs from aluminum.  
The downside of using PVC tubes for conventional speakers include the difficulty of the manufacturing process and cost of adding adaptations for stands, speaker mounting interfaces, speaker terminal interfaces and finishes.  
The higher end B&W diamond towers mimic this to some extent- they remind me of trash receptacles. 

The downside of using PVC tubes for conventional speakers include the difficulty of the manufacturing process and cost of adding adaptations for stands, speaker mounting interfaces, speaker terminal interfaces and finishes.

Its not really difficult. All you need to do is attach the tube to the driver. Takes about 2 minutes. You are wrong. 

from an engineering standpoint a tubular shape is inherently the most rigid and deflects the least under pressure than a box shape.  

It depends. A pvc tube can be bent easily by hand if you squeeze it across the width. But its harder to bend lengthways. The material matters more than shape. An mdf box is still more rigid than the pvc pipe used for the linkwitz speakers.