I have two s/510’s and spent the time fine tuning through the high level connection per REL’s philosophy once dialed in (it will take time) it was the most significant bang for buck upgrade I have ever made to my system. Once you have the speakers and the amplification sorted this should be the next investment. Don’t wasted thousands on power conditioners, interconnects etc. once you have decent quality cables (few hundred bucks) the diminishing returns are massive use that money on a single or pair of REL’s. Its not just base the impact on soundstage and imaging is material.
I’m not going to suggest REL make the only good subs but I will say don’t buy subs off speaker co’s just like you wouldn’t buy speakers off a company that specializes in subs. A sub is a different animal and different type of speaker that requires different R&D. Go REL, JL Audio etc. I’m going off script here a bit but needed to get that off my chest……