... Timfitz, can you tell me the differences between the 656 and the K66 since you have both? Interested in your perspective...
Glad to. I will caveat my response by saying mine is obviously a 'legacy' 656. If the re-voiced 656 follows the trend I've been hearing with the other guitars that I've sampled (614ce & 618e) in the new 600 Series, my observations will not really apply.
My 656 is pretty-much exactly what you would expect from Taylor's 'legacy' Maple guitar line: very bright and focused. The GS body still gives it satisfactory low-end 'rumble,' but largely the dominant characteristic (certainly by comparison to other Taylor 12-strings) is brightness; some might refer to it as 'jangly.' Personally, I like that characteristic in a 12-string, but others do not.
The K66 is noticeably more mellow-sounding than the 656; perhaps it's safe to say that it's a better-balanced 12-string than the 656. Koa still provides more brightness than most of the other tonewoods used by Taylor, but I think it's more centered on the midrange (as opposed to Maple's tendency toward the treble side); plus Koa naturally provides a bit more bass response than Maple, which gives it more 'warmth.' So I think the K66 will generally come across as a fuller-sounding 12-string.
Now, speculation time:
if the re-voiced 656 is anything like the new 614 and 618, I think it will more closely compare with the K66 than the 'legacy' 656 does. Taylor seems to have re-worked the 600 Series to effectively shift the tonal response of Maple toward the lower frequencies. Personally, I'm not a fan; while the bass response has been nicely enhanced and retains the clarity of Maple, the trebles sound 'clipped' to me. But again, others like it, so it's largely going to be a personal preference thing.
Good luck with your search! Let us know which 12-string you choose! Frankly, I don't think you can go too far wrong with either a 656 or a K66!