I have owned the fathom 113v2 with CR1 and the S510s. The companies obviously different philosophies. While JL's features are all about the frequency response, REL has videos suggesting positioning by way of impulse/timing in reference to the main speakers. That would be impossible with the fathoms. I am confused by REL's approach. Positioning subs to sync an impulse with main speaker would realistically mean the frequency response will not be optimal. Like Duke says, my experience is that peaks and dips and long decays are easily heard and measured. Their difference are also reflected in preferred connectivity. With my (balanced) amp, I much prefer the ease of low level inputs to the quirkiness of hi-level used by REL.
The fully automatic EQ included with the fathoms is powerful and does allow for more liberal placement than the RELs. I. did have personal satisfaction of getting the mains and rels just so, to even out 40Hz and below, though. Ultimately I needed a crossover to flatten a 60Hz vertical resonance from my main speakers.
It is hard to know what features are needed for both particular people and particular rooms/speakers.