I'm not sure how much this will help, but when I compared my EP500 to the PB13 Ultra, the difference was minimal until I played with the phase settings. Both subs were next to each other in the best spot in the room, set to 0 phase. I don't know why(well, the room I suppose), but the sound improvement was dramatic on the PB13 when the phase was changed to 180 degrees, a difference I couldn't replicate on the EP500. I don't know if phase experimentation would be worth a try in your case with 2 subs, but worth consideration perhaps?
I’m guess the reason you needed different phase settings for the EP500 and PB13 Ultra in the same place is the DSP circuitry in the EP500. Another thing I finally got a hard number on was that the DSP circuitry in my version of the EP500 v2 adds 8 feet to the physical distance. Prior to that I was only guessing based on a friend, who’s a part time sound engineer, calibrating it for me by ear. He said by the sound of it 8-9 feet extra was being added. Also the Onkyo bass management is adding 5 feet and Doug from SVS said that the EQ1 adds about 8 feet.
I did play with the phase settings when not using EQ. luckily the D-10 have a dial so I could get the phase as close as possible by ear. However, the EQ1 takes care of the phase setting when it’s hooked up and from the sound of it especially when blending with the mains it’s doing a great job of it.
I even try running them out of phase intentionally but that only made things worse.
