I too have a single REL T/7x that I am using with a pair of stand mounted Tonian Oriaco speakers. I found that corner placement worked the best just as REL suggests. The sub is to the left of my left-most speaker.
If you have a sub adjusted properly you shouldn’t “hear” it at all. You should simply hear more weight and lower overall extension. I use the warble tones from the Stereophile Test CD and a SPL app on my phone positioned at my most common listening position to find the smoothest response and best extension, but there are many ways to set-up a sub.
I enjoy the way the single REL T-7x enhances my listening. I hope you will too once you get it properly integrated for your room and speakers.