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.
Different brand/model Sub(s) integration
I have an all analog setup, the speakers are 2 Tannoy Kensington and one REL s/510.
I have a room mode at around 30Hz, that I can't fix regardless the REL's location. The only viable solution I can see is to add another sub, because the REL does not have DSP and no bass-trap can effectively absorb it.
Do I need to add another s/510 or I would be better off adding another brand with DSP so I can null the room mode?