Three different subs. Challenging to configure. I can see why you were having some cancellation with the EP500 and the Velo dls-5000R. Besides the usual placement issues, the 500 has a tuning point around 22hz while the Velo I believe is higher, around 28hz I guesstimate,--that spread is enough to create some problems with dialling in the phase between those two subs. Collocating makes more sense since you get the extra 6db "free" output from reinforcement to work with. The DLS-5000R is known to have high distortion below tuning point so collocating also has the added benefit of allowing you to lower gain on the Velo.
The DD-18 is a spectacular sub that does everything well. Congrats. I may sell the Ultra and buy one. At some point you may want to try the stereo sub alternative but you need two similar models for best results. My rig setup currently has two stereo EP600s and an Ultra for LFE and redirected surrounds. The 600s are equalized with a BFD 1124P equalizer and the Ultra with a SMS. Pretty linear response all around after many sweeps though trickier to configure than 3 LFE + redirected subwoofers. The reward is there for now. I notice the difference in panning street scenes where the cars/bus go from one side of the screen to the other. It sounds more natural and I believe it has to do with how the soundtrack was recorded and the timing of the bass from side to side. I'm the only one who notices though.
