I'm not so sure about headstock diameter and tone, as among the Guilds I own at present
only two, an '89 D50 SB and a '94 GV 70 are of the narrow variety. I've played both pre and post Grunhn-Walker guitars (I may be mistaken, but it seems that the change occurred on their watch) of the same model, and my wizened ears couldn't tell the difference.
I always thought that the "snakehead" headstocks were a bit of homage to Stromberg who used that design on their guitars. Maybe G-W were trying to start with a clean slate? Hans
might have some insight into it.
Again, all of this is really subjective, but I tend to like the more symmetrical narrow to wide headstock. It just looks correct and it's probably not an accident that they retained the "old style" on F50s and D55s.
David