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The Grand Auditorium designation derives from Martin's early naming conventions, with 0 size guitars referred to as 'Concert', 00 as Grand Concert, 000 as Auditorium, and 0000 as Grand Auditorium, so 16" width lower bout, non-dreadnought guitars tend to fall into the Grand Auditorium bucket, despite very different body shapes (e.g., Martin J and Gibson J-185).

The Taylor and the OLF SJ & MJ are based on the Gibson narrow-waisted jumbo shape of their early archtop instruments repurposed in the mid-1920's for the original Nick Lucas Special and L-0, then later, the J-185, J-200, etc. Taylor tends to flatten the neck attachment area of their guitars, which gives a boxier, less graceful shape than the Gibson originals, but made neck fitting with their original butted neck design easier and cheaper. Both of the OLF jumbos retain the more graceful curves of the original Gibson shapes, and show the distinctive changing lower bout radius which Taylor forgoes, again for the sake of easier production.

The Martin Grand Auditorium shape is significantly different - it is the shape of the 16" width Martin F-size archtops, which had fuller lower bout lines and the distinctive Martin waist and upper bout found on the 14 fret body shapes that the manufacturer designed in the late 1920's and early 1930's to replace their 12 fret-to-body models. The Martin J and M sized guitars are identical in shape, with the J built to dreadnought depth and the M built to the shallower 000 depth. More recently, Martin and a range of other small manufacturers have acknowledged the popularity of the narrow waist jumbo design by borrowing the key characteristics for their own takes on these classic Gibson body shapes.

Most Grand Auditorium (referred to as GA or 0000 as well) tend to produce more balanced tone (versus the bass-accentuated dreadnought shape of similar width but wider waist), but with more volume than the smaller 000. The shape can be used to build 12, 13, or 14 fret-to-body variants without much more than sound hole and minor bracing and bridge plate movement, as both Martin and Gibson did with other body shapes.

Appearance wise the Lowden "O" and the J185 are my favs -- they are very similar. Its my belief that the sound quality potential of the J185 is sacrificed a bit because of the short 24.75 scale.