A few things to consider.
I found the following posted elsewhere, I have not verified it.
"The fundamental of the low E of a bass guitar is 41.203 Hz.
The low A on a piano is 27.5 Hz.
Kick drum (floor toms and tympani) "fundamentals" can be anywhere from about 90 Hz on down to 25 Hz
depending on how the drum is tuned."
Due to way we hear frequencies you won't hear sub 30Hz frequencies unless you play your music really loud. Lookup "equal loudness curves".
I suspect you need near rock concert volume.
Speaker manufacturers exaggerate their specs. I just looked up the specification of a Klipsch RF7-II with 2 10" drivers specified as 30Hz -3db.
However, a posted frequency graph shows bass dramatically drop off starting at 40Hz.
Speakers compress their response as volume goes up. In other words, crank up the volume on the Klipsch RF7-II
and bass response may start to fall off dramatically at even higher frequencies.
I suspect if your main speakers really did 28Hz you would not benefit from a sub-woofer, but they likely do not.
Thus you probably want to cross-over higher, like 60Hz.
Despite REL's recommendation, I think you want to high-pass the main's and low pass the sub-woofer. I don't believe you can do that with the high level input of the REL that you have.
I would suggest that if you have a receiver that is capable of setting a cross-over point that high passes the mains and low passes the subs that you give that a try. You can usually
choose from several crossover points like 60, 80, 100, 120, 150. And a receiver likely has a remote control over volume adjustment to the sub-woofer.
Level matching is critical, takes a ton of time listening to a variety of music and is hard to do without a remote. And maybe impossible to do well without high passing the mains.
Room interaction is likely going to be a big problem with really low frequencies. You may need 2 sub-woofers carefully placed to help even the room response.