Being familiar with REW you should be able to accommodate any speaker. What causes people to experience room overload is simply the inevitable room modes. It’s the peaks that are responsible, peaks that can be 15/20 dB higher than the average in room SPL. These peaks will obviously take much longer to decay and this leads to the 3 complaints of: boomy bass, one-note bass or slow bass.
Get the room dialed in and that trio of problems disappears. Room treatment and a couple of small subs will sort out not only the bass but the full spectrum
The REL sub you own I think has phase flip only and this complicates matters in so far as finding the best position for it. Consider something like a pair of SVS SB1000 PRO or any other subs that provide variable phase adjustment so that you can place them where they are unobtrusive. or where convenient and not be restricted. It is better to have sealed subs and not ported because ports are frequency invariant and are bass sources that can’t be tuned. REW and mic are your best friends. Choose the speakers you desire and note that when the peaks are tamed and the nulls filled in you will be hearing much more than before and even at low SPL will sound great. Good Luck at the show.