The proper place to cross over an added subwoofer (and I've owned a bunch of Rels (over 20 years) is first to measure where the mains drop off in your room. Bass frequencies sum. So if your mains and sub are playing the same frequencies you'll often end up with a bass bump. So you'll want to set the crossover just below where your mains drop off.
Measuring does not have to be difficult or expensive. If you have a smartphone, Studio Six Digital has apps, both ones that cost money and free ones (including an RTA). Then you'll need pink noise and it can be downloaded (or bought). There are free downloads. Lastly, you'll need a mic for the smartphone. Parts Express sell the Dayton Audio IMM-6 mic (it used to be cheaper - think it is around $22). The mic comes with a custom calibration file that you download after you get it.
If you have a lapton and prefer to use REW (Room EQ Wizard, which is free) or another program, Parts Express sell a UMM-6 USB mic. It is more money but it depends on you preferences.