Though I don't know your room layout, I find it hard to believe that you can't add a second sub. Even if they aren't in a symmetrical placement (which, I think would actually be beneficial, as you could eliminate room nodes).
Some might say stacking, but for me, I wouldn't- once again, you might just aggravate room nodes.
I haven't looked at the differences between the two REL models, but considering that they should only be adding to the lower frequencies, I doubt the newer model will provide any meaningful improvement, especially at the added expense.
Given the fact that you currently enjoy your current system, I wouldn't bother with the sub upgrade. That $1500 could be applied elsewhere to more discernable improvements.
My 2 cents...
Bob