Is this how a Subwoofer Crossover is supposed to work?

I could be wrong, but it looks like you are running 2 subs in parallel with 2 mains (ie, mains are full-signal; subs are supposedly filtered out above the crossover frequency).

If I'm correct, the single most assured/competent resolution would be a good electronic crossover. I use the Marchand XM66 with variable crossover frequency. Slopes on the high- and low-pass crossovers are 24 dB/octave and the crossover "hump" (overlap) can be precisely dialed in. This is a very transparent, analog means to take a single RCA/single-ended input (from preamp) and split it into 2 output RCA pairs, one going to the subs (L goes to one powered sub, R goes to the other powered); and the other going to the amplifier that drives the main speakers.

It totally works. 

Again, thanks to those of you that are providing constructive input.

1) I've put the system back together, moving one sub to a different location and setting phase and crossover frequencies individually for each sub.  I am still using the DAC to provide fine tuning of volume with it connected by XLR inputs to the subs and by RCA to the tube amp, but now that I have a better understanding of what is going on the volume on the tube amp is set higher and the volume on the DAC is lower which helps offset the 7 dB gain of the XLR inputs vs RCA.  You can see a chart here. NOTE this was measured before adjusting Amp volume up and DAC volume down so the low end bass response isn't actually exaggerated in the system.

https://www.dropbox.com/s/j5a418zmk99zqbu/Moabs%20with%20Starke%20Subs%20Green%2C%20without%20Orange...

Adding the subs provides significant low end extension and a slightly smoother response.  I have significant room nodes at 61, 77, and 99 Hz with steep 10-15 dB valleys that I'm not sure what to do about.  The subs don't help with that and moving the Moabs around hasn't done much either.  I don't hear them so that is good.  Any suggestions about smoothing those room nodes?  I've tried a couple of rockwool panels without much impact.

2) Active crossovers or DSP all seem like good suggestions but things that I would explore at a later time.  I do use the EQ in Roon to adjust for some things but I haven't figured out how to use REW to measure the changes Roon makes.

3) My assumption has been using the DAC to control volume is fine and that there is no change in signal quality with volume set at 100 vs 60. Anybody have any idea if that is true?

4)  mijostyn's idea of subwoofer placement on either side of the speakers is interesting but, as many people do, I have my tube amp right in the middle.  It is on springs and isolation pads etc. etc. but it seems risky to have a tube amp in between, and close to, subwoofers.  Thoughts?

In conclusion, mostly I've roundtripped to where I started.  The system sounds great, the subwoofers extend the bass and increase low end impact without sounding exaggerated or noticeable.  At some point in the future I may explore active crossovers or DSP but not now.  I would still love to see measurements for how changing the crossover frequency works on a Rel or other high end subwoofer.

Yes! That i show adjustable sub crossovers should work, except for the volume problem.

That is how adjustable sub crossovers should work, except for the volume problem.

hogwash. Why would anybody design an electronic lowpass filter with a variable slope and a fixed crossover point? Thats useless. What you want is a variable crossover point and fixed slope. Even better would be both variable. 

kenjit "Why would anybody design an electronic lowpass filter with a variable slope and a fixed crossover point? Thats useless. What you want is a variable crossover point and fixed slope." 

That is what I thought also and that is what started this whole thread.  But Starke says the subwoofer is designed properly.  And I haven't been able to find any charts that show how other subwoofers respond with changes in the crossover.

Surely some enterprising soul on Audiogon has performed similar measurements on their sub and can post them or knows of a source showing how a "quality" subwoofer works?