Hi Ken
I have only tried one cantelivered neck guitar and I was not impressed with the sound, both in terms of timbre and sustain. Maybe it was just a bad one, but it definitly did not compare favourably with the many other guitars have tried in that particular price range ($8000 to $10 000), for what it's worth to compare guitars according to price...
On the other hand, almost every guitar with a Humphrey/Byers elevated fingerboard design I have tried showed increased sustain in the highs way up on the neck (past the 12 fret). I have built a few myself and I do notice the difference as well (I often take my finished guitars to a couple of professional player friends for a test drive, and they both concur). It has thus become a favourite model for me and I try to push client towards that as I think it makes a better sounding guitar.
The theory is that the increase mass under the fingerboard (where the neck continues rather than being cut on a standard guitar) helps sustain the vibrations a little longer and transfers some of those vibrations to the soundboard. I could be way off with this, but most of the other luthiers I discussed this with seem to agree. In the same vein, the cantelivered neck solution would negate this advantage since there is no contact between the upper neck and the soundboard. Slanting or bending the soundboard to accomodate the elevated fingerboard is also often cited as an advantage (by either increasing the tension or changing the angle of the strings over the soundboard), and having more space between the strings and the soundboard also accomodates players with a more aggressive attack (so that they don't hit the soundboard with their fingertips), another possible advantage. Both these last two points apply to either the cantelivered and "standard" elevated fingerboard designs.
My opinion is therefore that the Humphrey/Byers elevated fingerboard design provides both an advantage to the player with its easier access to the upper frets, and improves the sound performance of the guitar as well. For a couple of hours extra work required, me thinks it's a pretty good deal!
Cheers
Pat
