Hitting your original price range, Cambridge Audio S20.
This group is giving you good advice on subs, but open floor plan condo, I'm guessing neighbors as well. I think a small sealed 10", which will only go down to 30 Hz or so may be a safer bet. It won't put out huge volume, but I'm guessing you don't go to reference levels is that condo either.
I'm in a somewhat bigger condo and am just using a pair of full-range towers because if I had the sub from my last house, the neighbors would be knocking on the door.