Sub Confusion

Thanks to all for the valuable input. When I said I didn't need more bass, I meant what blisshifi said is exactly what I meant- "to clear up the frequency spectrum and deepen the soundstage". However, blisshifi, mentioned that other than the S812, the others don't dig deep enough. I know there's a difference between the S812 and S510; however they are both -6db, with the S812 at 19hz vs the S510 at 20hz. Given that, and based on my room size, wouldn't it be wise to start with a S510 as suggested by Matt M (setting the crossover at 65Hz)? 

Vandersteen sub3 w high pass filters if your integrated has pre out / power in. Better bass and mids….especially the mids….

@thebingster IMO that last Hz matters more than it appears. Those measurements  are at -6db. In reality, the S/812 is likely more like 24hz at -3db and the S/510 is probably 29hz at -3db. Both are not great measurements compared to many subs that dig much deeper, but Rel is known more for its speed than its ability to dig deep (aside from its reference products). 

Thanks @blisshifi for that info. And for clarification to others, the DarTZeel integrated does not have a pre out, so sub(s) will be connected to the main amp outputs. Looks like I should start with one S812, and save up over time for a second.

Another company offering subs with high-level hookup capability (the sub taking it’s signal off the power amp’s speaker binding posts) is Rythmik Audio, in their non-XLR plate amps. Those plate amps offer both high-level and line-level (from a pre-amp’s outputs) connections.

Available models include 12", 15", and 18" woofers, in both sealed and ported enclosures. All models employ servo-feedback circuitry.