Don't know a whole lot about the Guild bracing patterns... other than scalloped vs not. However, I noticed something else on my D-55 the other day that I plan to investigate further. I was messing around A/B/C-ing my D-55, D-50, and D-46 the other night... all three have good saddle height... but noticed there seemed to be less break angle on the D-55, tho' the saddle is about the same. Got out my caliper, and discovered that the bridge pin holes are set back farther, away from the saddle in D-55, different than all my other Guilds. Since the scales are the same, the saddles must be in the same relative position... but I wonder if the bridge plate inside is shifted rearward... and whether this might be a sign of a rear-shifted X-bracing in the D-55. I plan (but haven't yet) investigated farther.
One thing I would do is consider your bridge plate. The original bridgeplate of your D-25 was probably rosewood (at least that's what is i most of my Guild dreads). There are some folks that think a maple bridge plate, being less dense and thus less mass, gives a more "open" top than a rosewood. My D-35 and D-55 appear to have original maple plates... my D-44 has a maple plate, but I'm pretty sure it is not original. Another point... many mfctrs cut their bridgeplates with the grain parallel to the pin holes (across the guitar top; all my Guild except the D-44 are that way). This is not structurally very sound...as the weakest dimension/direction is in the direction of the most stress. Plus, the pin holes weaken it further. My D-44s bridgeplate has the grain running diagonally across the plate... and should resist stress and cracking at the pin holes better (Frank Ford has a discussion of this on Frets.com).
You might also want to look through
this thread on the UMGF... I have been thinking of trying this with my Guilds the next time I change strings, and we (LTG) might compose a library of Guild bracing patterns (maybe Hans has one already?)...
