Different brand/model Sub(s) integration

Hard to say astolfor. The ideal solution would be to get another s/510 and a stand alone two way crossover/room control unit like a MiniDSP. 

Get a trial period.

@mijostyn, would the mini just attack to the subs and then EQ from there?

TIA

I think you need more than one sub, at least 2, the more subs will reduce your dead zone. Absolutely use a crossover with a high pass filter to your main stereo speakers. 

@fiesta75 I am going to get one more, but the question is do I need another s/510 or I can use another brand or model and smaller?

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? 

@oldhvymec +1 on the servo sub suggestion. Once you go servo you can never go back!

I am a robot.

+2 oldhvy - No bottom firing subs and yes on servo subs if you can afford.

@astolfor , The MiniDSP goes between your preamplifier and amplifiers (the amps in the subs included)

Servos on subs are only necessary when you are asking drivers to do things they do not ordinarily do.  It is a sign of poor or undersized drivers. Large well designed subwoofer drivers have no problem maintaining distortion levels below 0.5%. You run into higher distortion levels when you start asking drivers to take long excursions. If a driver is taking excursions greater than 1/2 X max than it is too small for the job. You can either use larger drivers or multiple drivers. 

I've not heard sub that sounds as clean and crisp as my Rythmik servo sub. So if that is wrong I am very happy with it being wrong!

@mijostyn I want to keep everything as analog as I can, so DSP inline with everything else is not an option for me. I thought you were talking to make a crossover network among the subs.

Thank you for the suggestion 

Sell the Rel?...Thats a really stupid idea!...You own the best selling subwoofer in Rels lineup and it is  for a reason.Outside the reference #25 and the G2 you are there.I had the same issue with the same sub.I tweeked my room some and fix it a little...but when i bought another S510 thats when my system came alive..Yea you can go with another inferior subwoofer but at the end of the day thats exactly what you have.I would never take advise from someone who has no first hand experience with the product..cheers and happy tunes

@missioncoonery  do you want it for free 🤣 no worries I will not sell the REL... 😂. I have a room with 3 G1 MK II, and another room with 4 #25s in my actual home. 

In my house in the USA 2 510s will be tight that is why I was thinking something smaller and different response with the hope to help the room mode.

I am getting another 510.

Analog active crossovers are available.