Different brand/model Sub(s) integration

My suggestion would be to do a search in this forum for discussions about both "swarm" and "distributed bass array". This should point you in the right direction to not only solve your null at 30hz, but also improve the SQ immeasurably.

I'd go with another brand and or model. Different resonance frequencies are a good thing. For me it would be a larger one that goes down lower in frequency, but whatever you can afford would be an improvement. You're also going to need to experiment with various sub locations in your room. 

Yes get another and stay away from bottom firing subs, unless they have a butt plate or a fixed slot that doesn't use the floor. You CANNOT decouple that type of sub.. I quit using them in the 80s.

SVS or JL or GR OB servo subs. Sell the REL and buy a pair to start with. IBs ported or passive radiators just NOT firing downward with an open bottom.. Kid stuff OP, pure kid stuff.. I been building bass enclosures for 45 years..  No one around me uses them they are the blight of the neighborhood. Makes everything vibrate.. Even the kids are decoupling their subs and mass loading the tops with springs too.. 

I am not a robot

Regards

Probably better off different but only the robot knows for sure.

Have you placed the subwoofer at the listening position then walked the room listening for the loudest bass mode?