Hi Dave...great stuff!
A few comments:
1. Absolutely would agree that the 15" cones should be considered part of the main full-range design, not subwoofer territory. While i cannot recall in what hertz range those cones are performing...i would not class them as being driven within the X1 design as genuine SUB-woofer signal range.
2. In my own personal views of setup...NOTHING is perfect. EVERYTHING has pros and cons. The key is to max out on pros and minimize the cons. Sometimes you get no deterioration and only benefit...which is great...but still never PERFECT.
3. With that in mind, I find the single Velodyne DD18+ sub to work extremely well for 3 reasons:
a. I happen to be able to place it well within a reasonably large room...a good fundamental start
b. It is incredibly adjustable with 8-bar equalizers, phase control, crossover, volume, low-end roll off control (for sub-15hz problems), etc, etc
c. It is just big (no not rotary big but big)...and with a servo-control (3000 per second?), it is very tight and controlled while really driving serious air...someone said nearly twice the earlier generation 18" Velodyne sub??? Having owned both, i could actually believe that though i cannot say i have ever measured it
4. Would 2 Velodyne's work better? As long as I could handle the complexity of 2 subs being set up and properly intergrated...sure...and probably more so for 4 or even 6. The trouble is...it takes up a lot of real estate and starts to make the X1s into a 4-tower...fun for me, not so much for my wife.
5. As for how I did it with one:
See the 3 things above. In general, by placing it in a reasonably good spot (and frankly the only spot i could think of before we moved into/renovated the house)...i was off to a decent start. By then working over hours of the adjustments (manually), i got to a good position...and then literally over MONTHS, i would (perhaps 1/week, eventually 1/month) make a tweak here or there based on an album where i felt something was off and the album was allowing me to finetune by ear a bit here, and a bit there.
And now i dont touch it at all. Is it perfect? Nope...but i think for my skills/patience level, doing one works super well and i dont have the 2x-4x complexity of setup issues, space issues. And all day long, i would keep this versus having none.
Hope that helps and feel free to ask away. If i can be helpful, happy to do so.