"SVS SB-4000 is a worthy competitor at a fraction of the price and offers better integration with its mobile app."
Ummmm do SVS subs connect via the main amps outputs? I dont think so. the above statement will never be true, just because theres an APP? Thats not how subs integrate.
The REL subs connect through the main amplifiers outputs just like the main speakers for true integration and you dont do this using Hpass filters either!
If you need more than one sub your using the wrong subwoofer.
And to answer the OP's question YES the S510 will be plenty for your room size. I would buy it in a heartbeat. And make sure you set it up as REL reccomends for best bass response and musicality......YES REL's do more than produce bass they are musical and when tuned in properly they make the main speakers come alive even more and allow them to play with more detail and ease. REL subs sound beautiful.
Thanks
Matt M