I use Rel sub also. Most people set crossover to high. In your case the crossover at about 25 hz should work well. Sub in a system is not only deeper, tighter bass. It affects mids and treble also. There is more micro detail, more air, the soundstage is more open gents. When you turn off a sub during listen session you realize how much sound you miss without it.
Subwoofer Crossover Question
What would be the starting point for Subwoofer integration with a pair of speakers that has the Frequency response below. Any layman explanation for what the -6db at 23Hz signifies and how this might relate to subwoofer crossover into a two channel music system, (two Rel Strata III's) will also be appreciated.
Frequency Response 28Hz to 25kHz ±2dB, -6dB at 23Hz
Frequency Response 28Hz to 25kHz ±2dB, -6dB at 23Hz