Thanks to all the DIY'ers that have posted here, my inspiration came from the many contributions made to this forum.
Here is the final setup....and I'm very, very happy with the performance.
Dual Sonosubs, each with:
15" Adire Tempest driver (downward firing)
Dual 8 ohm VC's wired in parallel for 4 ohm load to amp
20" diameter sonotube, 4 foot length for volume ~214L (Adire Alignment)
48 oz polyfill stuffing
2 x 3/4" MDF inner disks
1 x 3/4" veneered plywood outer disk
2 x 3" flared vents
1 x dual input terminal cup
The bases were my wife's idea - 4 inch piece of sonotube with 1.5" holes cut out every 3 or 4 inches around the perimeter, covered with speaker grill cloth. It is very sturdy, looks clean and modern and does not seem to have any impact on sound quality that I can detect versus using traditional legs.
Amplifier: Behringer EP2500 (1 channel per sonosub)
Fed from LFE on my HK AVR145cost estimate:
$320 (2 x Tempests)
$70 (speaker carpet)
$80 (flared ports)$60
(glue, stain, nuts, bolts, misc)
So for under $1,000 I have a pair of subs that can compete with most commercial offerings costing much, much, much more.
.....and for how it sounds............simply awesome!
I don't have measurement equipment, but it sounds very clean for music (not overpowering) and when the LFE movie content kicks in and the EP2500 lights up , that's when it really gets impressive...so I know the low end performance is there.
I have a few cosmetics to clean up, but I'm pretty much done for now. Maybe in the future I can build a set of mains like the Nat P's that others have done.