BUT if I back the trussrod off by 1/4 turn the action rises a bit more than I’d like (but I can live with it), but (now here is what I can’t figure out) the tone becomes louder and bell like with incredible overtones and sustain. I know someone will know the science behind this.
It sounds like you realize that the truss is actually for adjusting neck straightness and that the slight rise (or lowering as the case may be) in action is only a side-effect or "fringe benefit".
I used to tweak my D25 a couple of times a year, too, and I did notice that when I tried it lower it a taste it lost some sustain.
Here's what I think is going on:
When the neck is given more backbow for flatness, it's actually under more tension and this turns it into a more effective
damper for the string vibrations.
Back off the truss a wee bit as you say, and it's not under as much tension and allows more string energy to be dissipated as sound instead of being absorbed into the neck at the nut, or maybe the heelblock, or both.
Here's what made me start thinking about those physics:
A couple of weeks after the D25 had its first refret at about 18 months old, it had what I thought was "the big opening up moment":
I was giving it a warmup-strumming workout when I noticed something special and I thought to myself "Wow this thing has never sounded this good before!" and I could literally
feel the neck vibrating in my hand.
I'd never noticed that in a guitar before.
I theorized that somehow the vibrations from the body were getting into the neck and the neck was putting 'em back into the top, through the fretboard extension, maybe?, in some kind of acoustic feedback loop.
And the truss was still set for just a tiny bit of forward-bow for clearance, as part of the step-up from the refret, although I didn't put 2 and 2 together about that until a few years later.
Another possibility is that by backing off 1/4 turn you're lowering the tension on the strings just enough to hit a sweet spot for sustain.
And maybe both.
Anyway, those're my hypotheses and I'm stickin' with 'em until something better comes along.
When I retire I'll have time to really scour the 'net for evidence to support all my pet hypotheses.
:biggrin-new: