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

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..

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

not really. it rings like a bell. 

Aluminum is actually one of your softer metals. Way down ⬇️ on the Mohs scale of hardness. 2.75 to be specific. It’s still an interesting material when thick, like 2-3”.

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..

which is why im saying its not good. 

At the same thickness PVC is similar in strength to MDF

We are talking about the linkwitz pluto speakers. They are not concrete pipes. They are not 2 inch thick PVC. They are PVC pipes that are about 2mm thick. I dont want to hear any more excuses. Do you know how bendy 2mm thick PVC is? Theres nothing rigid about it 

Actually...

The PVC pipes used would have a minimum wall thickness that well exceeds 6mm.

DeKay