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

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. 

PVC tubes are great for building Helmholtz resonators. There’s a simple formula for figuring out the resonant frequency Fr based on volume, nozzle length and nozzle diameter. I built a 60 Hz resonator out of 6” PVC pipe sections and PVC elbow joints. The trick is using the purple prep stuff just before applying PVC cement. My resonator was 15 foot long folded S shape.

At the same thickness PVC is similar in strength to MDF. With the "bend" forces exerted on a speaker enclosure, a tube is a significant advantage over a box making the comparative thickness higher but that would be compared to an unbraced box.

Aluminum, is a good material for low resonance, and great strength.

The problem is cost. Oh my. 

MDF
13-15 ply plywood, void free 3/4"
Hardwood
Aluminum

As far a cost, lowest to highest. 
Some hardwoods can exceed Aluminum for sure, but normally they are veneers.

There were a few companies that used PVC enclosures. Advent, 
Same era, Infinity was using cast Aluminum/magnesium at 10 times the cost, and 5 time the Retail. 

I'd like to say my speakers were CNC from marine aluminum, but paying for them, is a whole different story..

Here the thing though, cylinders, have an inherent characteristic to ECHO
and GAIN... not disappear, or blend like you want.. They're usually have pretty high mechanical distortion. Even with a lot of baffles..
Concrete pipe... ECHO, even long square tubes echo, and GAIN..

Big BASS Yes especially loading a tube inside a tube, and porting both enclosures. That's a 6th order, band pass. Home use.. OH my, you couldn't do it. Pictures would be falling off walls.

Columns... They work. Less real estate, less distortion, more bass, BASS, B A S S..