Oh I see what you're doing. Yea that's perfectly fine. Really the subs do not distinguish Left and Right, so technically you don't necessarily have to run to the corresponding channel on the high level inputs. If you want to run from the Left speaker out channel on your amp to the right high level input on the sub it doesn't make a difference.
Not too familiar with the Rolls SX45, but the from what I can tell, you are exactly right in terms of how you integrate the subs with that system.