I agree with Bojack...take your time and listen to a variety of music that you like...some source material will sound overwhelming in LF bass and other material will sound lean....adjust cut off and sub amplification level to give you the best balance for your room.
As a starting point, if your speakers are monitors ( no port ) then they will have a second order slope at the corner frequency or 3 DB point....if you Rel has the same filter setting in reverse (low pass second order)....then set your REL to about 10 to 30 Hz below this cut off. If your speakers have ports, then you will have a sharper cut off and you may set the sub closer to the 3 db point.
Alternatively you may use bass management (if you have this in a digital pre-ampilfier). Often you can use 4th order filters in this configuration and in this case you have even more flexibility with adjustments (although you will always be limited by the low end frequency reponse of your main speakers).
After tweaking adjustments and placement, a good match at a particular enjoyable sound level should almost always be achievable.
Unfortunately it will be a bit of a crap shoot to get sub and speakers to integrate together accurately at all sound levels as they are from different manufacturers and not necessarily designed to work together optimally. (Driver and cabinet design can affect how the speaker responds to increasing power levels. If the sub and speaker respond differently then adjustments to achieve a match at low SPL levels may be inappropriate at high SPL levels...again a compromise is needed.)
