Do No Harm ... well the least harm anyway

+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.

My 2 subs are VINO (Velodyne in name only) because, being Velodyne, they have both broken. Tired of repairing them, I bypassed the electronics and ran wires to just the drivers. So now they are 15" passive subs. I use a Crown 2500 for the lowpass (55Hz) and all is working fine. When I said "thumpy bass" I meant that in a good way. Like the sound from a kick drum, those frequencies are best suited to subs, not to a full-range driver. My preamp has 2 outputs (XLR and RCA) so the XLR goes to my Benchmark amp and the RCA to the Crown. Interestingly I have found that hooking the subs out of phase produces more bass than in phase. Even though the subs are close to the main speakers. Perhaps the Crown amp inverts phase, but I don't know. Everything sounds pretty nice right now, but as a hobbyist, I am always thinking of something to change.

The only way to do it with least detriment is digitally. And even then you’ll need to carefully choose the product. 

When I had to integrate a subwoofer to a high quality desktop audio system, I had already worn out the passive alternatives. I knew it was time to get serious, so I purchased a gently used Marchand XM66 electronic crossover. I have used this with great success for the past 10 years.

It has a variable (user selectable) crossover point, and both the high pass and low pass outputs going through a 24 dB/octave circuit. It sounds utterly clean, transparent, and unaltered as compared to no crossover.

It’s a steal. Of course, you could spend 4-5X the $$ on an electronic crossover by one of the audiophile names, but I"m not sure how much better it would be than the XM66.

https://www.marchandelec.com/xm66-electronic-crossover.html

Yes, The XM-66 is probably the highest on my list of possibilities.