There's many variations of Salicional, but the ones I've heard are generally moderately soft, not too bright. But then I have no Theater organ experience! They may be different. I would suggest perhaps scouting some of the free demos of Sonus Paradisi, like the Caen for example, which contain a Salicional and download it so you can hear it for yourself and compare. On the Caen, I would compare it to a softer version of the Gamba, but it's not a Theater organ.
~S