Full range is good for 5 channel/immersive music. My movie listening levels are a lot louder. Usually the low frequencies in the surround channels are rolled off at 80 Hz which allows the amp to have more reserve power for the LCR and the sub does what it is designed to do. Since your amp has 120 W for each of the 5 channels you can certainly give it a try.
The X-LS is -3db down at 55Hz so the crossover turns some 30 - 40Hz movie bass into wasted heat. Movies with strong deep base like "Blade Runner 2049" would probably bottom out the woofer if run full range at Dolby reference levels.
Are you listening to the X-LS off axis? How far do you sit from the surrounds? Surround speakers should have a wide even dispersion pattern, even going as far as bi-pole designs. The X-LS look like they are very focused, great for stereo music but for surrounds...?
X-LS Horizontal Response
I noticed a big jump in clarity, dynamics and surround effects after moving to the Anthem AVM 60 after my old Outlaw 975 died. The Outlaw was about as old as your Rotel RSP-1570 and the sound was definitely dated. I also discovered home theater upgrades aren't cheap.
One advantage of the new room correction software in modern processors are different speakers are tonally matched and volume corrected, in my setup it turns the center and 5 pair of slightly different sounding speakers into one huge seamless sound space. Sounds move around in space without any speaker localization. It helps that all the surround and Atmos speakers are bi-pole.
Here is an example of the frequency response adjustments I have at my fingertips.
