In a small room, I'd say the Spacials have a more recessed and slightly broader stage and a more neutral sound, as apposed to the Harbeths being a bit more forward and on the warm side. The sound of the Spacials (at least the older, M 3 Turbo S, model) remind me a bit of the smaller 1.7 Maggies (that I am using now) with a bit more base energy but with the same deep, broad stage. Like the Maggies, they have a full rich sound without being overly warm or bright.
All I know about the SALKs is the many good reviews and user claims of them being very room and Wife friendly, with the ability to produce a deep, wide stage and a clean, full but uncolored sound, with a minimal of room treatments and set up head aches.
I love my Maggies (ribbons and ESLs in general) but have a larger well dampened room and the patients it takes to set them up....Jim