I agree with @erik_squires. Integration is the key. And this is what Rel in known for. I used one Rel Storm 3 for years before upgrading to 2 S5's. The big consideration is to get the ones with enough power & size to fill the room. this was the biggest drawback with the one Storm. But the newer subs have a lot more amp than the earlier ones. I'd start with one of the T9's which has 300 watts vs 200 watts for the T7. Then there is the 10" woofer vs the 8" in the T7. You can get the second one later when the budget allows
Also, don't worry about the position of the passive radiator