A little X-bracing history
In the 20th and 21st centuries an X - brace design is generally associated with the steel-string guitar, but it was already being used by the Roudhloff brothers in London for their classical guitars back in the 1830s and 40s ! (bibl. "The century that shaped the guitar" by James Westbrook), and it is possible that later, Martin saw one of Roudhloff's X-braced guitars and subsequently adopted the idea (thanks to James Westbrook who has invesigated this area) and used a similar design in his 0-28 models that Julian Bream played in his youth. The Roudhloff's were russian emigrés who trained in France and then set up shop in London where they experimented with various designs, including the X-braced one. Jim Westbrook has discovered more information on the Roudhloffs' first X-braced, their so-called "Melophonic guitar" while studying for his phd in Cambridge.
It was after seeing a photograph of a C19th guitar with X-bracing for the top in James Westbrook's book that my interest in the design was piqued. In 2009 I built my X-braced prototype using model 12 shape with a cedar top that employed an elliptical sound-hole to allow positioning of the closed up X braces. This design was very successful and it occurred to me that it could work very well in the small- bodied model 08 (photo bottom right) with simplified strutting, where the long vibrating length of this design can be put to good effect in extending the bass response. Despite the small body, a low air resonance similar to a larger fan-braced guitar can be easily achieved with this X system - a small guitar with a deep bass and great projection is the result. You also get considerable weight of note and sustain with this pattern, as well as an even response and excellent balance. Having simplified the X-braced pattern for the 08 small-bodied shape, I went on to try it in the larger bodies ( 12 and 13) where the results were equally good. The X-brace works very well with cedar as well as spruce, which is an added bonus. I have also successfully used Port Orford cedar (a North American species of cypress) with it. I first used this pattern in the small body Torres 08 shape for a spruce/indian rosewood 640 mm scale guitar which is used by young students at the Yehudi Menuhin School.
For whatever reason, it was the spanish fan-braced pattern (as exemplified by Antonio de Torres), which was widely adopted for the classical guitar, while the X-brace was forgotten. Clearly there is nothing new about using an X-brace for classical or other nylon strung guitar types and there are good reasons for using it, as alluded to above- the long bars and greater working area of the top allow the maker to achieve a low body resonance with resultant good bass and excellent balance. The mass of the braces in the lower bout is greater than in the fan-strutted design and this combined with the longer vibrating length and larger working area result in more sustain.
Of course, in recent times the modern nylon strung guitar has undergone dramatic changes, both in the kind of sound produced and in the building methods and materials used, in an effort to produce ever louder guitars with greater projection. However, contemporary guitars are not to everyone's taste, and personally I prefer a more traditional approach and sound. The X-braced guitar benefits from very good sustain and immediacy. One could argue that an X-braced guitar is just as traditional as a spanish fan-braced one - it's just a different tradition, and as well as making an excellent crossover guitar, it is certainly a valid alternative for the classical guitar, and one worth exploring.