Do you run the Linns full-range? Most REL users seem to prefer this set-up, but in your case the Keilidhs may be overworked in such a large space. Before adding a second sub (which, as noted by all posters here will probably be a good idea, regardless), I'd try low-cutting the Linns with an active crossover. Even if you don't want to run the system this way long term, it will be a good diagnostic tool for you. The NHT x-2 active x-over is available for app. $300 with a money back trial period.
Good Luck
Marty
Good Luck
Marty