Saturday I made a trip from Maine to New Hampshire and back to try out the the limited edition 518, 618 and 918. I don't know that I can provide any meaningful comments as I find it very difficult to effectively convey tonal impressions but the guitars were impressive. It should be noted that I play without picks and keep my fingernails short so the sound I produce on these guitars might be quite different from those that most people achieve.
My judgement is that there is a greater difference between the grand orchestra and grand symphony than there is between the grand symphony and the grand auditorium. I first played the 518 and my immediate perception was that the sound was a bit pinched. That adjective may not mean much to most people and if I had to explain it differently I'd say that it seemed as if a band of midrange frequencies were attenuated as compared to any Taylors I've played that I can recall. (A sound spectrum analysis could very well show that what I'm calling a pinched sound may be due to something else but that is the way I envisioned it as I played.) That perception persisted when I played the other two guitars. When I tried the 618, I quickly put it back and went on the play the 918. Initially, I thought I preferred the 518 but when I went back to it, I realized that I liked the 918 more. (That I played the 618 between the 518 and the 918 may have been a factor.)
I had my Engelmann/Brazilian rosewood grand auditorium with me in case I wanted to trade it in towards any of the orchestra models and for comparison purposes. (I've taken that guitar to Gruhn Guitars in Nashville and to an R. Taylor roadshow but never found something for which I was willing to trade it in.) When I switched from the 918 to my guitar, I again noticed that pinched quality but as I switched back and forth, I decided that the net effect of the 918 was preferable to the overall sound of my guitar. My wife also preferred the sound of the 918. A guitar is a product of compromises and it is artful compromise that yields a good guitar. I decided to trade-in my grand auditorium guitar but couldn't reach an understanding with the dealer so I returned home without a new guitar.
I just got off the phone with the dealer and we have struck a bargain so I'll be saying good by to a guitar I've owned for 16 years and will be picking up the 918 on Saturday. On that day I will own Taylors made in 1996, 1997, 2001, 2003 and 2013.