+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.