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.
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?