I agree, generally not recommended. I had two different subs both 12", I calibrated using AudX32, but they didn't totally blend, but it was adequate/acceptable. Finally got a matching sub and it sounds/blends much better (no sound localization). Hope this helps.
Peace and blessings,
Marantz SR7011: 7.4.4 Martin Logan Edges (4: FR/FL/SR/SL) Martin Logan Axis (2: SBL/SBR) Martin Logan Stage X (Center) Martin Logan 1500X Sub (2) Martin Logan 1100X Sub (2) Sonance VP86R SST/SUR (4: Dolby Atmos) Epson 6040UB/Elite Lunette 2 120"