Thanks, gang! That answers it.
Keep on pluckin'!
Actually they all missed the
real point:
Be aware the D55 was simply the name given to D50's which Tommy Smothers ordered as "D50 Specials":
D50's fitted with F50 necks, when they finally decided to make it a model in its own right (Special order only from '68-74, then became regular production)
At the core at introduction in '64, the D40 and the D50
were identical in build and materials quality except for the body woods.
After that the models went their different courses of development but the true apples-to-apples comparison would be between D40 and D50, not D40 and D55.
As Dreadnut mentioned build era is relevant too.
If you're talking
current production then the true apples to apples comparison is between the D40
Traditional and the D55, primarily because they both have the real dovetail neck joint and 3-pc necks and HG NCL finish.
The most significant difference between 'em is that the D40 has a "high grade solid sitka top", the D55 gets a AAA sitka top.
Grading is supposed to be a primarily cosmetic issue but there is a school that believes that the tighter more consistent grain associated with "AAA" grading
could make better soundboards.
I'm still on the fence because my limited sampling doesn't support that belief.
My own '03 D40 with AA top is the top of the heap now, tone-wise.
D-55s have a Mahogany neck with a central Walnut stripe, whereas the D-40 has just a Mahogany neck (one piece I should imagine)...The reinforced D-55's neck should be more steady; I think that's why they do it this way, but can't tell about efficiency
It
is supposed to also help resist "twist" as you supposed.
...I once had a D-40 and the neck was very steady, not the slightest problem on that one...
Mahogany is already considered to be a very dimensionally stable wood in environments of changing humidity so wasn't sure why they'd make a 3-pc out of it until I realized twist is a different issue from humidity.
I didn't realize that the D-40 build formula had changed that much--I don't recall ever seeing a one-piece neck on an older (20th-century) D-40--a three-piece laminated neck was a consistent feature, as it still is on the Traditional line.
Yeah it actually was already gone by '72, don't think it was ever revived until New Hartford's Bluegrass Jubilee version (my '03 Corona has a one-piece).
.
How you feel about the voice of a D-40 vs a D-50 is a personal matter--I never cared for what rosewood did for the D-50 or any big-body guitar, but individual instruments can easily challenge that general impression. (My current favorite guitar is a rosewood Goodall Standard.) The rest of the formula is mostly cosmetic, though there are those who hear differences in the variety of spruce used for the top or even fingerboard material (I'm not among them).
I have no rosewood experience either, but funny thing, over the last few days I listened to some recordings made back around 2012 on the D40 with
a really good microphone and heard what I'd started thinking I wanted from rosewood: really sweet, clear, ringing highs, like when holding a note at the end of a scale, and downright thumpy bass.
Had forgotten it could sound so sweet.