I haven't ever played any of them with the maple back. This is good to hear. Thank you. I did not realize that the maple back was arched.
Like Tom said, not all of 'em are arched, the visual tell is whether or not it has back bracing. (the F55Maple is arched just like the original F50. In fact Guild's very first flattop was the F50, with maple arched back.)
An arched back will never have bracing, it doesn't need it.
Guild currently doesn't offer an American-built maple arched back dread, but when they did there were only a couple with the "bling" level of the D55 such as the Hank Williams G45 (2nd iteration).
Much more common was the D30 (originally the G37 from '74 to ca '86 when the D30 "replaced" it.) I mention the D30 because I suspect it'll be the easiest arched maple back dreadnought to be found for sale/tryout, currently. They also had ebony fretboards and AA tops, making 'em the closest to D55 specs you're likely to find.
Re the flat vs arched back sound, regardless of the woods involved, the arched back form enhances sustain and overtones. The shape also serves as a parabolic reflector towards the soundhole which enhances volume.
The varying depth means a greater spectrum of frequencies are being processed inside the body as well, this is where the enhanced overtones come in. That can have a price though, as things like frequency interference/cancellation can "muddy up" the sound.
Works good for chord work, makes 'em sound full and lush like Tom said, especially with maple and 'hog, but with rosewood's already strong overtone generation it can be overkill. I think that's why Guild only ever built 2 regular production guitars with arched rosewood back and only 2 more "special order" models.
In comparison there were more arched maple back models than flatback, and Guild's all-time best seller the D25 had an arched
mahogany back for the vast majority of its production life.
A flatback renders much tighter more focused sound, it's said bluegrass pickers would prefer it because having a shorter duration of sustain, the notes won't tend to interfere with each other when playing arpeggios and leads. Less phase interference/cancellation occurs inside a flat-back body.
I once made the comparison that an arched back is like a parachute flare, lights up everything but the light's "diffused" and details are a little blurry, whereas a flatback is like a spotlight: tight and focused and shows up details well.
Took me a while to figure it out, while getting to know my D40 (flat 'hog back) after playing nothing but the D25 (arched hog back) for several years. Those 2 guitars have essentially identical build formulae except for the back configuration, so a true apples-to-apples comparison.
When I play down on the E & A strings on the '25, I can get a woody tone similar to that of a stand-up bass. When I do it on the D40, it's more like a Fender P-bass type of sound.
I also have a maple arched-back model in the F65ce and in fact a couple of years ago I started experimenting with ways to tone down the oft-mentioned maple "jangle". Not so evident to me from the playing position, but definitely noticeable on cell-phone recordings.
