Adding a sub to usher be10

The Ushers go down to 25hz. Thats pretty deep.
If you go with some subs, the crossover frequencies will have to go down pretty low to blend seamlessly.
You may cause a hump at the 30hz range if not careful.
The 1% of music that goes below 25hz may not be worth the effort.
You may cause more neg. than pos. in your quest.
Just my 2 cents worth.

I have a difficult room also with a wall of windows and 10' ceiling. I have struggled to get good bass output with many speakers. Adding a REL subwoofer ended my problems. I say go for it.

You don't mention what it is specifically you're trying to accomplish here, especially since you're already using the Lyngdorf for room correction duties. That said, there are not many speakers on the market today at any price that wouldn't benefit at least somewhat from two good-quality subs using good integration software (not sure if your Lyngdorf can handle two subs?). I'd be surprised if you didn't hear a fairly meaningful positive difference -- and not only in the bass range -- if you follow that path. Then again, it won't be a cheap upgrade either. Best of luck.

Thanks to all, I will first complete some work on the room, then probably shop for a pair of subs!

Rythmik audio? Genesis?

Regards
Charles

W