This is a tough one. I have a Kawai digital piano, too (really nice keyboard feel) and I've used Carver cinema ribbons run full range with a sub for the bottom end. The Carvers roll off naturally at about 90 cycles in my room and are IMHO one of the few speakers that hands off nicely to a sub without an active crossover. I ran the RCA outs to the subwoofer and the full range output from the sub to the Carvers. I also used a corner bass trap between the piano and the corner it straddled.
This isn't a set-up I'd use for hi fi reproduction, but it worked very well for this application. OTOH....
While the sound is "crisper" and more dynamic than the internal electronics in the Kawai, you lose the resonance of the piano's "box" and the feel that comes back to you thru the instrument. These days, I generally use the internal set-up, although the external system is still there. On the rare occasions that my wife or daughter play for an audience (something I'd never do) we may activate the external system.
Good luck.
This isn't a set-up I'd use for hi fi reproduction, but it worked very well for this application. OTOH....
While the sound is "crisper" and more dynamic than the internal electronics in the Kawai, you lose the resonance of the piano's "box" and the feel that comes back to you thru the instrument. These days, I generally use the internal set-up, although the external system is still there. On the rare occasions that my wife or daughter play for an audience (something I'd never do) we may activate the external system.
Good luck.