Aerial Acoustics, the speaker firm that Michael Kelly conceived a quarter-century ago with David Marshall, is headquartered north of Boston, not far from the Merrimack River Valley region that once produced textiles and shoes by the trainload. Kelly, though, is quick to equate Aerial with far more distant firms. His industrial models are in Germany, where he lived for a while when his father, a US Army officer, was based there, and where he later spent time as a vice-president of a/d/s/, which had been founded by a German-born and -educated scientist, Godehard Günther, who died last October. They're small-to-midsize specialty firms that together constitute a category called Mittelstanden, and they're as accomplished as they are narrowly focused. They're artisan enterprises, and it's only natural that someone as dedicated as Kelly is to building state-of-the-art loudspeakers would embrace them as examples.