Would adding another REL STRATA Help?

if your model is the Strata III that unit only has an 10" driver you mght be better with using two larger Rel subs or another model that can support a large room. Two Rel stratas might work in fact it will help support more power and dynamics...also please cover that glass wall and your ceiling's are high so absortion panels will help.

Yes, REL recommends two when possible, I use two Stadium subs i a much smaller room.

I'd always use minimum of two subs regardless of room size. So yes, go for another! place one of them near center of one side wall. The other wherever you get smoothest response.

Rockadanny is right. Two subs are always better than one.

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