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?