I also have an all-Koa, Grand Symphony 12-string (the K66ce), and am very happy with it. Compared to my Sitka/Maple 656ce, I'd say they're similar in overall tonal range, but I would agree with "BigSkyTaylorPlayer" that the 656ce is much brighter-sounding (some refer to it as "jangly"). I guess the best way I've found to describe it is that the K66ce is a mellow-sounding 656ce.
When I was shopping for a Koa-bodied 12-string, I had initially intended to get the Sitka-topped version (the K56ce). But it was recommended to me that the K66ce would be the better guitar to purchase, perhaps based on the relative popularity of the two models. When I inquired what the primary tonal difference between the K56ce and K66ce was, I was told that the K56ce would essentially be somewhat louder in volume, but not too different in tone. Never having had a chance to compare them, I can't say if what I was told was correct, though. Anyway, I followed the advice I was provided, purchased a K66ce, and haven't regretted it at all.