Do No Harm ... well the least harm anyway

Bomer:

I had the DBX Venue 360 in the system, but found it to be annoying having to run it through my computer. It was supposed to run on an iPad also, but they never fixed the problem that arose when Apple changed their OS a while back. I also did not like the additional A-D and D-A that had to be incorporated. I have been considering the Mini DSP Studio, or the Marchand passive crossover, but after a bunch or reading it seems this whole idea is a can of worms that I would probably have wished I never opened.

Hello, It looks like your subs have a 100 hertz high pass filter. Just run your line to the input on the DD15’s and then out to your amp. You might have to look at a manual but there is a 90 or 100 crossover switch on the back. You can at least use this to see if you like the sound or if it degrades the sound in any way. 

If those speakers have “Thumpy” bass it’s either your room or most likely the position of the speakers.  Probably they’re too close to the front and side walls.  
Before you try surgery or software, I’d start with pulling them three feet from the front wall.  That’s three feet from the back of speakers.

All the best.

+1 @soix

With speakers going down to 26Hz (not including the room), I would think you would start with the subs crossing over around 30Hz and play with the gain first, and then adjust +/- 5Hz and dial in from there. 

IMO phase should be set to zero as your subs are next to your speakers. 

And definitely play around with speaker placement. It is amazing how much a few inches in distance in any direction and 1/2 inch more or less toe in (or none) makes.

Lastly, what connection method are you using? I don't know what Velodyne recommends for 2 channel but my experience with REL is high-level from the speaker taps works best.