The REL T5/x is exactly 12" tall.
If your main speakers are rated to 22Hz I don't think a sub is needed and one that small may not help.
As others have stated maybe an amp that's rated for 1 or 2 ohms with sufficient current/power for your needs?
Maybe your room is the problem.
I'm running two REL T7/x subs to the left and right of the main speakers and don't have a problem. FWIW my room is symmetrical, rectangular, small (11x14) and treated.